chore: update actions/checkout to version 6 in phpunit workflow

Author: vinkla

.github/workflows/phpunit.yml

@@ -18,7 +18,7 @@ jobs:
     name: PHP ${{ matrix.php }}
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}