[Spark 4] Streaming runner proof of concept, unbounded sources, windowed GBK and stateful ParDo on transformWithState - #39576

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