build(deps): bump actions/checkout from 5 to 6 (#543)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/linux.yml

@@ -30,7 +30,7 @@ jobs:
             rdkafka_versions: { min: '>= 0.12.0', max: '= 0.15.2' }
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }} with rdkafka gem version (min ${{ matrix.rdkafka_versions.min }} max ${{ matrix.rdkafka_versions.max }})
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}