Small cmake cleanups of things no longer needed due to dep version bumps
- Drop explicit setting of CMAKE_AUTORCC, duplicated from KDECMakeSettings
- Drop check for no longer supported older Qt version
Related question: any plans to bump the min required cmake version to >= 3.11 soonish, e.g. indirectly by relying on KF >= 5.83 (which implies cmake >= 3.16)?
That would allow to remove all the "" from the add_library() & add_executable() calls.