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/main.yml

@@ -34,7 +34,7 @@ jobs:
       OROCOS_BUILD_TYPE: ${{ matrix.orocos_build_type }}
       ROS_PYTHON_VERSION: ${{ matrix.python_version }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
       - uses: actions/setup-python@v6
@@ -88,7 +88,7 @@ jobs:
             branch: release-1.5
     env: ${{ matrix.env }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
         if: ${{ (github.event_name == 'push' && endsWith(github.ref, matrix.branch)) || (github.event_name == 'pull_request' && endsWith(github.base_ref, matrix.branch)) }}