Bump actions/upload-artifact from 5 to 6 (#312)

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/build-and-publish-antora.yml

@@ -31,7 +31,7 @@ jobs:
         run: antora --fetch antora-playbook.yml
 
       - name: Upload site as artifact
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: site
           path: build/site

.github/workflows/gradle-build.yml

@@ -75,7 +75,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: ./gradlew check --configuration-cache --parallel
       - name: Upload build artifacts
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         id: upload-build-artifacts
         with:
           name: build-artifacts