Qt6/PySide6 compatibility + cleanup (re-apply of #439) - #461

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: tlambert03/setup-qt-libs@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/