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/default-tests.yml

@@ -7,7 +7,7 @@ jobs:
     name: Run checkstyle
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
       - name: Set up JDK 11
@@ -33,7 +33,7 @@ jobs:
       matrix:
         java_version: [ '11', '12', '13', '14', '15', '16', '17', '18', '19', '20' ]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
       - name: Set up JDK ${{ matrix.java_version }}
@@ -58,7 +58,7 @@ jobs:
     if: ${{ github.event_name != 'pull_request' && !startsWith(github.ref, 'refs/tags/') && github.ref == 'refs/heads/master' }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
       - name: Set up JDK 11