AdminEmailWorker only send repository check emails, so it belongs to
source code management. (It also doesn't run on GitLab.com.)

GitlabUsagePingWorker belongs to the collection category.

PruneOldEventsWorker relates to events which are used on the user
profile, so I've gone with users. This could also go under audit events,
potentially.

AdminEmailsWorker sends an admin-defined email to all users, so I put it
under issue tracking as that currently owns notifications (!). (This
also doesn't run on GitLab.com.)