Mk/bsd.sites.mk: Update GNOME

• Remove deprecated and/or dead mirrors, Gnome solely relies on a CDN now [1]
• Drop GitLab "hack", this is not a mirror and distfiles aren't static / match up with release tarballs Ports that needs to use GitLab instance should use USE_GITLAB

1: https://discourse.gnome.org/t/gnome-mirroring-system-updates/25031

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D50957