Update subosito/flutter-action action to v2

Author: renovate[bot]

.github/workflows/flutter-test.yml

@@ -13,7 +13,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
-      - uses: subosito/flutter-action@v1.5.3
+      - uses: subosito/flutter-action@v2.7.1
         with:
           channel: 'stable'