Update actions/checkout action to v6

| datasource  | package          | from | to |
| ----------- | ---------------- | ---- | -- |
| github-tags | actions/checkout | v4   | v6 |


Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/continuous-integration.yml

@@ -25,7 +25,7 @@ jobs:
       fail-fast: false
       matrix: ${{ fromJSON(needs.matrix.outputs.matrix) }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Prepare LDAP test server
         if: contains( matrix.job, 'vendor/bin/phpunit' )
         shell: bash