Update actions/checkout action to v3

Author: renovate-bot

.github/workflows/flutter.yml

@@ -20,7 +20,7 @@ jobs:
         channel: [ 'stable', 'beta' ]
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - uses: subosito/flutter-action@v1.5.3
         with: