[postmarketos-ui-gnome-mobile] Replace karlender with gnome-calendar

Gnome Calendar is Gnome Core app and (after Gnome 43) very adaptive, so replace Karlender (which also awesome) with it.

Blocked on GNOME Calendar getting CalDAV support. Needs https://gitlab.gnome.org/GNOME/gnome-online-accounts/-/merge_requests/98 first