Update subosito/flutter-action action to v2 (#21)

Co-authored-by: Renovate Bot <[email protected]>

Author: renovate[bot]

.github/workflows/flutter.yml

@@ -22,7 +22,7 @@ jobs:
           distribution: 'zulu'
           java-version: '11'
 
-      - uses: subosito/flutter-action@v1
+      - uses: subosito/flutter-action@v2
         with:
           channel: ${{ matrix.flutter-channel }}