Migrate new feature issue template to GitHub forms (#123682)

Fixes https://github.com/flutter/flutter/issues/119919

<details>
<summary>Preview</summary>

![image](https://user-images.githubusercontent.com/50555895/232455733-2c2cba5e-6a35-4e4a-b911-e0ef80746905.png)

</details>

.github/ISSUE_TEMPLATE/3_feature_request.md

----
-name: Feature request
-about: Suggest a new idea for Flutter.
-title: ''
-labels: ''
-assignees: ''
-
----
-
-<!-- Thank you for using Flutter!
-
-     If you are looking for support, please check out our documentation
-     or consider asking a question on Stack Overflow:
-      * https://flutter.dev/
-      * https://api.flutter.dev/
-      * https://stackoverflow.com/questions/tagged/flutter?sort=frequent
-
-     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. Please read
-     our guide to filing a bug first: https://flutter.dev/docs/resources/bug-reports
--->
-
-## Use case
-
-<!--
-     Please tell us the problem you are running into that led to you wanting
-     a new feature.
-
-     Is your feature request related to a problem? Please give a clear and
-     concise description of what the problem is.
-
-     Describe the alternative solutions you've considered. Is there a package
-     on pub.dev/flutter that already solves this?
--->
-
-## Proposal
-
-<!--
-     Briefly but precisely describe what you would like Flutter to be able to do.
-
-     Consider attaching something showing what you are imagining:
-      * images
-      * videos
-      * code samples
-
-     Does this have to be provided by Flutter directly, or can it be provided
-     by a package on pub.dev/flutter? If so, maybe consider implementing and
-     publishing such a package rather than filing a bug.
--->

.github/ISSUE_TEMPLATE/3_feature_request.yml

+name: Feature request
+description: Suggest a new idea for Flutter.
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thank you for using Flutter!
+
+        If you are looking for support, please check out our documentation
+        or consider asking a question on Stack Overflow:
+
+          - https://flutter.dev/
+          - https://api.flutter.dev/
+          - https://stackoverflow.com/questions/tagged/flutter?sort=frequent
+  - type: checkboxes
+    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
+  - type: textarea
+    attributes:
+      label: Use case
+      description: |
+        Please tell us the problem you are running into that led to you wanting
+        a new feature.
+
+        Is your feature request related to a problem? Please give a clear and
+        concise description of what the problem is.
+
+        Describe the alternative solutions you've considered. Is there a package
+        on pub.dev/flutter that already solves this?
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Proposal
+      description: |
+        Briefly but precisely describe what you would like Flutter to be able to do.
+
+        Consider attaching something showing what you are imagining:
+         * images
+         * videos
+         * code samples
+
+        Does this have to be provided by Flutter directly, or can it be provided
+        by a package on pub.dev/flutter? If so, maybe consider implementing and
+        publishing such a package rather than filing an issue.
+    validations:
+      required: true