Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/.ci.yml

@@ -9,7 +9,7 @@ jobs:
         php-versions: ['8.1', '8.2', '8.3', '8.4']
         os: [ubuntu-latest, macos-latest, windows-latest]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Use Node.js ${{ matrix.node-version }}
         uses: actions/setup-node@v6
         with: