Make improvements to existing new issue templates (#140142)

Closes https://github.com/flutter/flutter/issues/139558

In this PR:
- Use `<details open>` to create opened collapsible sections (still be expanded if needed)
- Change checkboxes (at the top of the issue, namely `I have searched the existing issues` and `I have read the guide to filing a bug`) to text only.

### Demo filed issues for `opened collapsible sections`

- [I am having difficulty installing Flutter or getting it to work](https://github.com/huycozy/flutter/issues/8)
- [Report a bug](https://github.com/huycozy/flutter/issues/9)
- [Report a bug in one of Flutter's first-party packages](https://github.com/huycozy/flutter/issues/11)
- [My app is slow or missing frames](https://github.com/huycozy/flutter/issues/12)
- [My app has some non-speed performance issues](https://github.com/huycozy/flutter/issues/10) 

### Demo for `checkboxes to text`

![Screenshot 2023-12-14 at 17 54 44](https://github.com/flutter/flutter/assets/104349824/1472b3bf-6245-4ec9-b7e8-d7ddf3d5a579)

- [I am having difficulty installing Flutter or getting it to work](https://github.com/huycozy/flutter/issues/new?assignees=&labels=&projects=&template=1_activation.yml)
- [Report a bug](https://github.com/huycozy/flutter/issues/new?assignees=&labels=&projects=&template=2_bug.yml)
- [Feature request](https://github.com/huycozy/flutter/issues/new?assignees=&labels=&projects=&template=3_feature_request.yml)
- [The CI infrastructure used by Flutter has a problem](https://github.com/huycozy/flutter/issues/new?assignees=&labels=team-infra&projects=&template=6_infrastructure.yml)
- [Report a bug in one of Flutter's first-party packages](https://github.com/huycozy/flutter/issues/new?assignees=&labels=&projects=&template=9_first_party_packages.yml)

.github/ISSUE_TEMPLATE/1_activation.yml

@@ -17,15 +17,12 @@ body:
 
         If you have found a bug or if our documentation doesn't have an answer
         to what you're looking for, then fill out the template below.
-  - type: checkboxes
+  - type: markdown
     attributes:
-      label: Is there an existing issue for this?
-      description: Please search to see if an issue already exists for the bug you encountered.
-      options:
-      - label: I have searched the [existing issues](https://github.com/flutter/flutter/issues)
-        required: true
-      - label: I have read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)
-        required: true
+      value: |
+        Before filling the form fields, please consider the following:
+        - Ensure that you have searched the [existing issues](https://github.com/flutter/flutter/issues)
+        - Read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)
   - type: textarea
     attributes:
       label: Steps to reproduce
@@ -58,7 +55,7 @@ body:
         Note: Please do not upload screenshots of text. Instead, use code blocks
         or the above mentioned ways to upload logs.
       value: |
-        <details>
+        <details open>
         <summary>Logs</summary>
 
         ```console
@@ -74,7 +71,7 @@ body:
       description: |
         Please provide the full output of running `flutter doctor -v`
       value: |
-        <details>
+        <details open>
         <summary>Doctor output</summary>
 
         ```console

.github/ISSUE_TEMPLATE/2_bug.yml

@@ -14,14 +14,12 @@ body:
           - https://flutter.dev/
           - https://api.flutter.dev/
           - https://stackoverflow.com/questions/tagged/flutter?sort=frequent
-  - type: checkboxes
+  - type: markdown
     attributes:
-      label: Is there an existing issue for this?
-      options:
-      - label: I have searched the [existing issues](https://github.com/flutter/flutter/issues)
-        required: true
-      - label: I have read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)
-        required: true
+      value: |
+        Before filling the form fields, please consider the following:
+        - Ensure that you have searched the [existing issues](https://github.com/flutter/flutter/issues)
+        - Read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)
   - type: textarea
     attributes:
       label: Steps to reproduce
@@ -62,7 +60,7 @@ body:
         Note: Please do not upload screenshots of text. Instead, use code blocks
         or the above mentioned ways to upload your code sample.
       value: |
-        <details><summary>Code sample</summary>
+        <details open><summary>Code sample</summary>
 
         ```dart
         [Paste your code here]
@@ -77,7 +75,7 @@ body:
       description: |
         Upload any screenshots or video of the bug if applicable.
       value: |
-        <details>
+        <details open>
         <summary>Screenshots / Video demonstration</summary>
 
         [Upload media here]
@@ -99,7 +97,7 @@ body:
         Note: Please do not upload screenshots of text. Instead, use code blocks
         or the above mentioned ways to upload logs.
       value: |
-        <details><summary>Logs</summary>
+        <details open><summary>Logs</summary>
 
         ```console
         [Paste your logs here]
@@ -112,7 +110,7 @@ body:
       description: |
         Please provide the full output of running `flutter doctor -v`
       value: |
-        <details><summary>Doctor output</summary>
+        <details open><summary>Doctor output</summary>
 
         ```console
         [Paste your output here]

.github/ISSUE_TEMPLATE/3_feature_request.yml

@@ -12,15 +12,12 @@ body:
           - https://flutter.dev/
           - https://api.flutter.dev/
           - https://stackoverflow.com/questions/tagged/flutter?sort=frequent
-  - type: checkboxes
+  - type: markdown
     attributes:
-      label: Is there an existing issue for this?
-      description: Please search to see if an issue already exists for this feature request or proposal.
-      options:
-        - label: I have searched the [existing issues](https://github.com/flutter/flutter/issues)
-          required: true
-        - label: I have read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)
-          required: true
+      value: |
+        Before filling the form fields, please consider the following:
+        - Ensure that you have searched the [existing issues](https://github.com/flutter/flutter/issues)
+        - Read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)
   - type: textarea
     attributes:
       label: Use case

.github/ISSUE_TEMPLATE/4_performance_others.yml

@@ -39,7 +39,7 @@ body:
 
         Note: Please do not upload screenshots of text. Instead, use code blocks or the above mentioned ways to upload your code sample.
       value: |
-        <details><summary>Code sample</summary>
+        <details open><summary>Code sample</summary>
 
         ```dart
         [Paste your code here]
@@ -90,7 +90,7 @@ body:
 
         Note: Please do not upload screenshots of text. Instead, use code blocks or the above mentioned ways to upload logs.
       value: |
-        <details><summary>Logs</summary>
+        <details open><summary>Logs</summary>
 
         ```console
         [Paste your logs here]
@@ -103,7 +103,7 @@ body:
       description: |
         Finally, paste the output of running `flutter doctor -v` here, with your device plugged in.
       value: |
-        <details><summary>Doctor output</summary>
+        <details open><summary>Doctor output</summary>
 
         ```console
         [Paste your output here]

.github/ISSUE_TEMPLATE/5_performance_speed.yml

@@ -39,7 +39,7 @@ body:
 
         Note: Please do not upload screenshots of text. Instead, use code blocks or the above mentioned ways to upload your code sample.
       value: |
-        <details><summary>Code sample</summary>
+        <details open><summary>Code sample</summary>
 
         ```dart
         [Paste your code here]
@@ -71,7 +71,7 @@ body:
 
         If the trace are too large to be uploaded to GitHub, you may upload them as a `zip` file or use online tools like https://pastebin.com to share it.
       value: |
-        <details><summary>Timeline Traces JSON</summary>
+        <details open><summary>Timeline Traces JSON</summary>
 
         ```json
         [Paste the Timeline Traces here]
@@ -88,7 +88,7 @@ body:
 
         Don’t use "adb screenrecord", as that affects the performance of the profile run.
       value: |
-        <details>
+        <details open>
         <summary>Video demonstration</summary>
 
         [Upload media here]
@@ -148,7 +148,7 @@ body:
 
         Note: Please do not upload screenshots of text. Instead, use code blocks or the above mentioned ways to upload logs.
       value: |
-        <details><summary>Logs</summary>
+        <details open><summary>Logs</summary>
 
         ```console
         [Paste your logs here]
@@ -161,7 +161,7 @@ body:
       description: |
         Finally, paste the output of running `flutter doctor -v` here, with your device plugged in.
       value: |
-        <details><summary>Doctor output</summary>
+        <details open><summary>Doctor output</summary>
 
         ```console
         [Paste your output here]

.github/ISSUE_TEMPLATE/6_infrastructure.yml

@@ -13,12 +13,10 @@ body:
         It looks like you found an issue with our Infrastructure services.
         Please complete the form below so that we can help to resolve your
         issue as quickly as possible.
-  - type: checkboxes
+  - type: markdown
     attributes:
-      label: Is there an existing issue for this?
-      options:
-      - label: I have searched the [existing infra issues](https://github.com/flutter/flutter/issues?q=is%3Aopen+is%3Aissue+label%3Ateam-infra)
-        required: true
+      value: |
+        Before filling the form fields, please ensure that you have searched the [existing infra issues](https://github.com/flutter/flutter/issues?q=is%3Aopen+is%3Aissue+label%3Ateam-infra)
   - type: dropdown
     attributes:
       label: Type of Request

.github/ISSUE_TEMPLATE/9_first_party_packages.yml

@@ -13,14 +13,12 @@ body:
           - https://flutter.dev/
           - https://api.flutter.dev/
           - https://stackoverflow.com/questions/tagged/flutter?sort=frequent
-  - type: checkboxes
+  - type: markdown
     attributes:
-      label: Is there an existing issue for this?
-      options:
-      - label: I have searched the [existing package issues](https://github.com/flutter/flutter/issues?q=is%3Aopen+is%3Aissue+label%3Apackage)
-        required: true
-      - label: I have read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)
-        required: true
+      value: |
+        Before filling the form fields, please consider the following:
+        - Ensure that you have searched the [existing package issues](https://github.com/flutter/flutter/issues?q=is%3Aopen+is%3Aissue+label%3Apackage)
+        - Read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)
   - type: dropdown
     id: packages
     attributes:
@@ -115,7 +113,7 @@ body:
         Note: Please do not upload screenshots of text. Instead, use code blocks
         or the above mentioned ways to upload this.
       value: |
-        <details><summary>pubspec.lock</summary>
+        <details open><summary>pubspec.lock</summary>
 
         ```lock
         [Paste file content here]
@@ -164,7 +162,7 @@ body:
         Note: Please do not upload screenshots of text. Instead, use code blocks
         or the above mentioned ways to upload your code sample.
       value: |
-        <details><summary>Code sample</summary>
+        <details open><summary>Code sample</summary>
 
         ```dart
         [Paste your code here]
@@ -179,7 +177,7 @@ body:
       description: |
         Upload any screenshots or videos of the bug if applicable.
       value: |
-        <details>
+        <details open>
         <summary>Screenshots / Video demonstration</summary>
 
         [Upload media here]
@@ -201,7 +199,7 @@ body:
         Note: Please do not upload screenshots of text. Instead, use code blocks
         or the above mentioned ways to upload logs.
       value: |
-        <details><summary>Logs</summary>
+        <details open><summary>Logs</summary>
 
         ```console
         [Paste your logs here]
@@ -214,7 +212,7 @@ body:
       description: |
         Please provide the full output of running `flutter doctor -v`
       value: |
-        <details><summary>Doctor output</summary>
+        <details open><summary>Doctor output</summary>
 
         ```console
         [Paste your output here]