Update actions/checkout action to v6

Author: text-html-renovate[bot]

.github/workflows/test.yml

@@ -25,7 +25,7 @@ jobs:
           - '8.3'
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2