Update GitHub Actions workflow to actions/checkout@v4

Author: TimWolla

.github/workflows/tests.yml

@@ -29,7 +29,7 @@ jobs:
 
         steps:
             -   name: Checkout
-                uses: actions/checkout@v2
+                uses: actions/checkout@v4
 
             -   name: Setup PHP
                 uses: shivammathur/setup-php@v2