php.yml: Update actions/checkout to v6

Author: reedy

.github/workflows/php.yml

@@ -21,7 +21,7 @@ jobs:
           - "8.4"
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Setup PHP with XDebug
         uses: shivammathur/setup-php@v2
         with: