Bump actions/download-artifact from 5 to 6 (#300)

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/build-and-publish-antora.yml

@@ -48,7 +48,7 @@ jobs:
       url: ${{ steps.deployment.outputs.page_url }}
     steps:
       - name: Download site artifact
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: site
           path: build/site

.github/workflows/gradle-build.yml

@@ -123,7 +123,7 @@ jobs:
           build-scan-terms-of-use-url: "https://gradle.com/terms-of-service"
           build-scan-terms-of-use-agree: "yes"
       - name: Download build artifacts
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           artifact-ids: ${{ needs.gradle-build.outputs.build-artifacts }}
       - name: Publish Sonar report