games/freeciv: Remove games/ggz-gtk-client dependency and unlock build on 13.0

ggz-gtk-client currently fails to build on 13.0. ggz-gtk-client
seems to be abandoned upstream and fixing it might not be worthwhile.
Freeciv does not really need it, so remove the dependency on it.

Also take maintainership.

PR: 238256
Approved by: johans (maintainer timeout, 2 weeks)