js: Update for Gio/GioUnix split

js: Update for Gio/GioUnix split

GLib 2.79.2 split out the platform specific Gio APIs into a separate
GIRs, while still keeping a copy in the Gio GIR. gjs then added warnings
about the APIs being moved.

This bumps the Gio(Unix) version requirement to 2.79.2 and switches to
using the new GioUnix GIR.

See: https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3892
See: https://gitlab.gnome.org/GNOME/gjs/-/merge_requests/918
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7539
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3288>