build(deps): bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up JDK
         uses: actions/setup-java@v4

.github/workflows/docs.yml

@@ -35,7 +35,7 @@ jobs:
     if: ${{ !(github.event_name == 'workflow_dispatch' && inputs.wiki_only == 'true') }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0    # Full history — needed to access labs/* branches
 
@@ -78,12 +78,12 @@ jobs:
     if: ${{ always() && (needs.deploy.result == 'success' || needs.deploy.result == 'skipped') }}
     steps:
       - name: Checkout main repo
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           path: main
 
       - name: Checkout wiki
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           repository: ${{ github.repository }}.wiki
           path: wiki