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

Author: gardener-ci-robot

.github/workflows/vendor_gardener.yaml

@@ -9,7 +9,7 @@ jobs:
     name: Run make tidy
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: actions/setup-go@v6
       with:
         go-version-file: go.mod