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

Author: renovate[bot]

.github/workflows/flutter.yml

@@ -10,7 +10,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - uses: subosito/flutter-action@v2
         with: