Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/continuous-integration.yml

@@ -21,7 +21,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: "Install PHP with extensions"
         uses: shivammathur/setup-php@v2