testing/libqofono: upgrade to 0.121, add qt6 build
Feedback requested, not sure if this package split is good:
- libqofono-qt5
- libqofono-qt6
- libqofono-dev (containing headers for both qt5 & qt6)
Alternatively this would also work:
- libqofono-qt5
- libqofono-qt5-dev
- libqofono-qt6
- libqofono-qt6-dev
Not sure which one is better.