Update Mac download page for Gitlab migration.
Update git.gnome.org and bugzilla.gnome.org URIs to point at gitlab.gnome.org, remove an ancient link to a long-since removed web forum, and modify the bug reporting instructions to match gitlab.
Update git.gnome.org and bugzilla.gnome.org URIs to point at gitlab.gnome.org, remove an ancient link to a long-since removed web forum, and modify the bug reporting instructions to match gitlab.