Change the build system to enable building with Qt6
This was built with:
-DQT_MAJOR_VERSION=6
-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=5.90.0
-DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x055a00
QTextCodec isn't needed with Qt6, as the default encoding for QTextStream is UTF-8 by default in Qt6.
All unittests still pass.