Bump subosito/flutter-action from 2.18.0 to 2.21.0

Bumps [subosito/flutter-action](https://github.com/subosito/flutter-action) from 2.18.0 to 2.21.0.
- [Release notes](https://github.com/subosito/flutter-action/releases)
- [Commits](subosito/flutter-action@v2.18.0...v2.21.0)

---
updated-dependencies:
- dependency-name: subosito/flutter-action
  dependency-version: 2.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

.github/workflows/release.yml

@@ -69,7 +69,7 @@ jobs:
 
       # Setup Flutter
       - name: Setup Flutter
-        uses: subosito/flutter-action@v2.18.0
+        uses: subosito/flutter-action@v2.21.0
         with:
           flutter-version: ${{ env.FLUTTER_VERSION }}
           channel: 'stable'