Improve running git gc during repository cleanup

What does this MR do?

Without these changes, housekeeping may not make some objects unavailable immediately, and disk space used by the repository may go up instead of down.

We've marked the repository as read-only for the duration of the operation, so it is safe to do a git gc --prune=24.hours.ago (added as the :prune action)

Once this is in, we may want to return to using git gc --prune=now for maximum effect, but that's a Gitaly-side change. Being read-only, again, makes this safe enough.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Part of #220104 (closed)