Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...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] <[email protected]>

Author: dependabot[bot]

.github/workflows/gradle.yml

@@ -25,7 +25,7 @@ jobs:
     env:
       MATRIX_JOBS: 5
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - id: set-matrix
       run: |
         node .github/workflows/matrix.js
@@ -40,7 +40,7 @@ jobs:
     env:
       TZ: ${{ matrix.tz }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Set up Java ${{ matrix.java_version }}, ${{ matrix.java_distribution }}
         uses: actions/setup-java@v5