Change the build system to enable building with Qt 6

This was built with: -DQT_MAJOR_VERSION=6
-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=5.90.0
-DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x055a00

All unit tests still pass, except for the ktwofinger* ones which require QTouchDevice, the latter isn't available in Qt6.