chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/main.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Flutter
         uses: subosito/flutter-action@v2
@@ -36,7 +36,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Flutter
         uses: subosito/flutter-action@v2