Bump subosito/flutter-action from 2.12.0 to 2.15.0

Bumps [subosito/flutter-action](https://github.com/subosito/flutter-action) from 2.12.0 to 2.15.0.
- [Release notes](https://github.com/subosito/flutter-action/releases)
- [Commits](subosito/flutter-action@2783a3f...3d80492)

---
updated-dependencies:
- dependency-name: subosito/flutter-action
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/flutter_markdown.yaml

@@ -32,7 +32,7 @@ jobs:
 
       # Install the Flutter SDK using the subosito/flutter-action GitHub action.
       - name: install the flutter sdk
-        uses: subosito/flutter-action@2783a3f08e1baf891508463f8c6653c258246225
+        uses: subosito/flutter-action@3d804929922b667a63a229bc59037807f969e885
         with:
           channel: 'beta'