Upgrade GitHub actions checkout to v4 (#108)

Author: odan

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Set up PHP ${{ matrix.php }}
         uses: shivammathur/setup-php@v2