Update actions/checkout action to v3 (#25)

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

Author: renovate[bot]

.github/workflows/flutter.yml

@@ -15,7 +15,7 @@ jobs:
         flutter-channel: [ 'stable', 'beta' ]
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - uses: actions/setup-java@v3
         with: