ci(dependabot): bump actions/upload-artifact from 5 to 6 (#487)

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v5...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] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -69,7 +69,7 @@ jobs:
           PYTEST_ADDOPTS: ${{ matrix.resolution == 'lowest-direct' && '-W ignore' || '' }}
 
       - name: Upload coverage
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: covreport-${{ matrix.os }}-py${{ matrix.python-version }}-${{ matrix.backend }}-${{ matrix.resolution }}
           path: ./.coverage*