Enable auto `git gc` on GitLab repositories created as mirrors
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### Proposal
As mentioned in this Ultimate customer's [ticket](https://gitlab.zendesk.com/agent/tickets/348236) (internal link), mirrored repositories on SaaS are hitting the max repository size limit, which is preventing them from running housekeeping tasks on the repositories without intervention from support.
The proposal then is to enable automatic `git gc` on mirrored repositories that hit this limit, or enable housekeeping to work even if the repository has hit or exceeded the max repository size on SaaS.
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