chore(deps): update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/flutter.yml

@@ -20,7 +20,7 @@ jobs:
         channel: [ 'stable', 'beta' ]
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - uses: subosito/flutter-action@v2.6.1
         with: