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/release.yml

@@ -12,7 +12,7 @@ jobs:
       - name: Python dependencies
         run: python -m pip install git-archive-all
       - name: Clone
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
            submodules: recursive
       - name: Archive code including submodules

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
         shell: bash -el {0}
     steps:
       - name: Clone repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           submodules: recursive
       - uses: conda-incubator/setup-miniconda@v3
@@ -28,7 +28,7 @@ jobs:
       - name: Test import
         run: python -c "import pygotm"
       - name: Clone gotm cases
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: gotm-model/cases
           path: cases