Remove references to bugzilla in wiki
The Gnome Shell home page (as referred, for example, by Debian/Ubuntu packages) https://wiki.gnome.org/Projects/GnomeShell is still linking to the old Bugzilla tracker. It's true that the Bugzilla tracker refers to Gitlab, bit since the wiki home page is to be reached by people considering reporting a bug up or downstream, it would be nice to facilitate things.