Use non-deprecated KDEInstallDirs variables (at ECM 1.3.0)
See https://invent.kde.org/frameworks/extra-cmake-modules/-/blob/v1.3.0/kde-modules/KDEInstallDirs.cmake
If you are fine with bumping the min ECM version to 1.6, we could also switch to KDE_INSTALL_TARGETS_DEFAULT_ARGS (which was my initial motivation to look here, to ensure nothing uses that enymore ;) ).