Update cherry-pick issue template to more uniform labels. (#128333)

There are some snake case labels (issue_link) and some sentence case labels (Commit Hash).

This creates inconsistencies in the visual output of the template and when creating automations.

.github/ISSUE_TEMPLATE/7_cherry_pick.yml

@@ -9,7 +9,7 @@ body:
   - type: input
     id: issue_link
     attributes:
-      label: issue_link
+      label: Issue Link
       description: What is the link to the issue this cherry-pick is addressing?
     validations:
       required: true
@@ -33,7 +33,7 @@ body:
   - type: input
     id: pr_link
     attributes:
-      label: pr_link
+      label: PR Link
       description: >-
         Link to an open PR that cherrypick's this into the target release branch.
         The current branches can be found under release-caniddate-branch.version for [beta](https://github.com/flutter/flutter/blob/beta/bin/internal/release-candidate-branch.version) and [stable](https://github.com/flutter/flutter/blob/stable/bin/internal/release-candidate-branch.version)