Update actions/checkout action to v3

Author: renovate-bot

.github/workflows/dart.yml

@@ -21,7 +21,7 @@ jobs:
         sdk: [ 2.12.0, 2.15.0, stable, beta, dev ]
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Setup Dart
         uses: dart-lang/[email protected]