Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/dart.yml

@@ -15,7 +15,7 @@ jobs:
       image:  google/dart:latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
     - name: Install dependencies
       run: pub get
     - name: Run tests