🍒 kcms/icons: filter out GNOME's Adwaita and High Contrast icon themes

These are no longer FDO-compatible icon themes, and are apparently no longer intended to be used for non-GNOME apps--going against what an FDO icon theme is supposed to be used for.

I've contacted the GNOME folks about it, and unfortunately they've made it clear that the situation is intended; see https://gitlab.gnome.org/GNOME/adwaita-icon-theme/-/issues/288

As such, allowing the user to select these icon themes will just give them an opportunity to break all their non-GNOME apps. Let's filter these themes out to prevent that possibility.

We still need to figure out a solution for when our apps are run in distros where Adwaita is used as the default icon theme, but that's a separate topic.

BUG: 486409 FIXED-IN: 6.0.5 (cherry picked from commit 813d22ff)