Change Maintainer email for all Debian GNOME packages

[Jeremy Bicha <jbicha@debian.org>]

According to the schedule [1], all Alioth emails will stop accepting
new mail February 1, less than 8 weeks from now.

Since we already have a lists.debian.org email address, I propose we
switch the Maintainer email address for the Debian GNOME packages from
pkg-gnome-maintainers@lists.alioth.debian.org to
debian-gtk-gnome@lists.debian.org.

For many packages, we can make this change when we change the Vcs
fields to git, hopefully this month or next.

[1] https://wiki.debian.org/Alioth#News

Thanks,
Jeremy Bicha