docs: Add use of Closes: tag for closing gitlab issues

This replaces to old Bugzilla: tag, which no longer makes sense because we don't use bugzilla anymore.

This makes the most sense to me, as having gitlab automatically close issues is a super useful feature, it also automtaically creates issue <-> MR links, so that if someone has an issue they can see that there's an MR or vice-versa. Using Closes makes sense to me since it's fairly obvious what it does, and it doesn't conflict with the Fixes: tag we already use.