ci(deps): bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  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/tests.yml

@@ -271,7 +271,7 @@ jobs:
         uv run sphinx-build -nW -b html -d docs/_build/doctrees docs docs/_build/html
     
     - name: Upload documentation artifacts
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: documentation
         path: docs/_build/html/