Set up Java 21 in workflow (required by Jena 6)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: namedgraph

.github/workflows/publish-docs.yml

@@ -20,6 +20,12 @@
        - name: Checkout LinkedDataHub-Apps
          uses: actions/checkout@v4
 
+       - name: Set up Java 21
+         uses: actions/setup-java@v4
+         with:
+           distribution: temurin
+           java-version: '21'
+
        - name: Install Linux packages
          run: sudo apt-get update && sudo apt-get install -y xmlstarlet