Bump subosito/flutter-action from 2.7.1 to 2.21.0

Bumps [subosito/flutter-action](https://github.com/subosito/flutter-action) from 2.7.1 to 2.21.0.
- [Release notes](https://github.com/subosito/flutter-action/releases)
- [Commits](subosito/flutter-action@v2.7.1...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] <[email protected]>

Author: dependabot[bot]

.github/workflows/flutter.yml

@@ -16,7 +16,7 @@ jobs:
         with:
           java-version: 11
           distribution: temurin
-      - uses: subosito/flutter-action@v2.7.1
+      - uses: subosito/flutter-action@v2.21.0
         with:
           cache: true
       - name: Version
@@ -33,7 +33,7 @@ jobs:
         with:
           java-version: 11
           distribution: temurin
-      - uses: subosito/flutter-action@v2.7.1
+      - uses: subosito/flutter-action@v2.21.0
         with:
           cache: true
       - name: Format