Only schedule updates of remote mirrors once per push

What does this MR do?

Before we were scheduling the update once for each pushed ref through the Git::(Branch|Tag)PushService. But all the refs will be pushed every time, meaning that most of these updates would be no-ops.

This moves the updated out into the PostReceive worker, so this is only scheduled once per push.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content