11.0/trisquel-meta: replace gnome-system-tools for mate-user-admin.

Due LP#1706770 ubuntu-mate removed gnome-system-tools and in newer releases adds mate-user-admin as the default tool.

Ubuntu Mate 22.04 has no defined tool to manage users, but on our case it has been added it by a couple of MR:

• package-helpers!1227 (merged)
• package-helpers!1226 (merged)

This MR adds mate-user-admin and force the removal of gnome-system-tools in trisquel (mate) metapackage.