Bump actions/setup-python from 5 to 6 in the ci group (#190)

Bumps the ci group with 1 update: [actions/setup-python](https://github.com/actions/setup-python).


Updates `actions/setup-python` from 5 to 6
- [Release notes](https://github.com/actions/setup-python/releases)
- [Commits](actions/setup-python@v5...v6)

---
updated-dependencies:
- dependency-name: actions/setup-python
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: ci
...

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

Author: dependabot[bot]

.github/workflows/antsibull-build.yml

@@ -77,7 +77,7 @@ jobs:
 
       - name: Set up Python 3.13
         id: python
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version: '3.13'
 

.github/workflows/antsibull-docs.yml

@@ -64,7 +64,7 @@ jobs:
 
       - name: Set up Python 3.13
         id: python
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version: '3.13'