Use non-deprecated KDEInstallDirs variables
See https://invent.kde.org/frameworks/extra-cmake-modules/-/blob/v5.19.0/kde-modules/KDEInstallDirs.cmake for them being defined with the current min required ECM 5.19-
See https://invent.kde.org/frameworks/extra-cmake-modules/-/blob/v5.19.0/kde-modules/KDEInstallDirs.cmake for them being defined with the current min required ECM 5.19-