Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
         steps:
             -   id: checkout
                 name: Checkout
-                uses: actions/checkout@v4
+                uses: actions/checkout@v6
 
             -   id: setup_php
                 name: Set up PHP version ${{ matrix.php }}