ensure Qt6 compatibility

[t0b3]

qgis plugins need to ensure they are compatible with Qt6 to continue to work with future qgis releases

see https://lists.osgeo.org/pipermail/qgis-developer/2024-January/066417.html

• There's a script available in the repo for automatically upgrading QGIS
  plugins/scripts to Qt 6 compatibility WITHOUT BREAKING compatibility with
  older releases/qt5 based releases. It's here:
  https://github.com/qgis/QGIS/blob/master/scripts/pyqt5_to_pyqt6/pyqt5_to_pyqt6.py

• After running the script, you'll also need to add "supportsQt6=yes" to
  your plugins metadata configuration. Otherwise the Qt6 builds will just
  ignore your plugin and will refuse to load it. See eg
  wonder-sk/qgis-first-aid-plugin@3687be7