NO-TICKET: Bump Java 21 to Zulu 21.0.12.1-21.52 (fixes openjdk-jre High CVEs) - #51

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, docker/setup-buildx-action@v3, docker/setup-qemu-action@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/