luci-app-ssr-plus: Improve dashboard panel management and add panel chooser. - #2048

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, actions/upload-artifact@v4, docker/build-push-action@v6, 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/

The 'tool-cache' input is deprecated; it was renamed to 'preinstalled-runtimes'. It still works and will keep working until v3.0.0 — please rename it when convenient.