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/tests.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       -   name: 'Checkout code'
-          uses: actions/checkout@v4
+          uses: actions/checkout@v6
       -   name: 'Setup Docker'
           uses: docker/setup-docker-action@v4
       -   name: 'Debug versions'
@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       -   name: 'Checkout code'
-          uses: actions/checkout@v4
+          uses: actions/checkout@v6
       -   name: 'Setup Docker'
           uses: docker/setup-docker-action@v4
       -   name: 'Debug versions'