php.yaml: use actions/checkout@5 (#239)

Author: reedy

.github/workflows/php.yml

@@ -19,7 +19,7 @@ jobs:
           - '8.3'
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v5
 
     - name: Setup PHP
       uses: shivammathur/setup-php@v2