Bump subosito/flutter-action from 2.20.0 to 2.21.0 (#13)

Bumps [subosito/flutter-action](https://github.com/subosito/flutter-action) from 2.20.0 to 2.21.0.
- [Release notes](https://github.com/subosito/flutter-action/releases)
- [Commits](subosito/flutter-action@395322a...fd55f4c)

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

Author: dependabot[bot]

.github/workflows/build.yml

@@ -20,7 +20,7 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
 
       - name: Install Flutter and Dart SDK
-        uses: subosito/flutter-action@395322a6cded4e9ed503aebd4cc1965625f8e59a # v2.20.0
+        uses: subosito/flutter-action@fd55f4c5af5b953cc57a2be44cb082c8f6635e8e # v2.21.0
         with:
           channel: "stable"
           cache: true