Build: upgrade actions/checkout to v4

- intentionally not using SHAs here to avoid more future work

Closes gh-459

Author: timmywil

.github/workflows/php.yml

@@ -19,7 +19,7 @@ jobs:
     name: Lint PHP ${{ matrix.php-version }}
     steps:
     - name: Checkout
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
 
     - name: Setup PHP version
       uses: shivammathur/setup-php@v2