CI: update actions/checkout to v6 in tests workflow

Author: bashgeek

.github/workflows/tests.yml

@@ -25,7 +25,7 @@ jobs:
           PEBBLE_VA_ALWAYS_VALID: "1"
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up PHP ${{ matrix.php }}
         uses: shivammathur/setup-php@v2