Update actions/checkout action to v6

| datasource  | package          | from | to |
| ----------- | ---------------- | ---- | -- |
| github-tags | actions/checkout | v5   | v6 |

Author: renovate[bot]

.github/workflows/continuous-integration.yml

@@ -18,7 +18,7 @@ jobs:
     name: PHPUnit Coverage
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: shivammathur/[email protected]
         with:
           php-version: ${{ env.default_php }}
@@ -44,7 +44,7 @@ jobs:
     runs-on: "ubuntu-latest"
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: shivammathur/[email protected]
         with:
           php-version: ${{ env.default_php }}
@@ -62,7 +62,7 @@ jobs:
     runs-on: "ubuntu-latest"
 
     steps:
-      - uses: "actions/checkout@v5"
+      - uses: "actions/checkout@v6"
       - uses: shivammathur/[email protected]
         with:
           coverage: "xdebug"