Bump subosito/flutter-action from 2.18.0 to 2.21.0 (#14)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/release.yml

@@ -49,7 +49,7 @@ 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'