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 17
@@ -44,7 +44,7 @@ jobs:
           '26'
         ]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
       - name: Set up JDK ${{ matrix.java_version }}
@@ -104,7 +104,7 @@ jobs:
             gradle_version: '7.6.6'
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
       - name: Set up JDK ${{ matrix.java_version }}
@@ -118,7 +118,7 @@ jobs:
       - name: Setup TEMP_DIR
         run: echo "TEMP_DIR=$(mktemp -d --tmpdir=/home/runner/work/git-commit-id-gradle-plugin/git-commit-id-gradle-plugin git-commit-id-gradle-debugging.XXXXXXXXXX)" >> $GITHUB_ENV
       - name: Checkout git-commit-id-gradle-debugging to run actual gradle integration tests
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           submodules: recursive
           repository: git-commit-id/git-commit-id-gradle-debugging