Update to actions/checkout@v4

Author: mgruner

.github/workflows/ci.yml

@@ -29,7 +29,7 @@ jobs:
         php: ['7.4', '7.3', '7.2', '8.0', '8.1', '8.2', '8.3', '8.4']
     name: PHP ${{ matrix.php }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Install PHP
         uses: shivammathur/setup-php@v2
         env: