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] <support@github.com>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -38,7 +38,7 @@ jobs:
       matrix:
         python-version: ['3.9', '3.10', '3.11', '3.12']
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: mamba-org/setup-micromamba@v2
         with:
           environment-name: wradlib-data-dev

.github/workflows/pypi-release.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.repository == 'wradlib/wradlib-data'
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: actions/setup-python@v6