setup-desktop: install wayland-base on KDE Plasma

KDE Plasma has been Wayland-compatible for a long time and with the release of 6.0 it's the default session. Since just after Alpine 3.21 was released I even removed X11 support from Plasma entirely because I have no interest maintaining it and people should use Wayland instead.

This reduces the installed (and useless) packages and still works as expected.