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
KDEInstallDirs must be included before the first Qt find_package(), so that it detects Qt5 by default.
It's been deprecated since KF 5.0. Use QProcess::startDetached() instead. And remove the dep on KService, not needed any more.
Edited by Ahmad Samir