Extract push mirror update logic into a service

What does this MR do and why?

Contributes to #455517 (closed)

Problem

Currently, push mirror update logic is included in Projects::UpdateService. It's error-prone and inconvenient to use push mirrors.

Solution

• Add RemoteMirrors::UpdateService
• Replace push mirror update code with a call to RemoteMirrors::UpdateService

How to verify

1. Enable feature flag use_remote_service_update_service
2. Try to update a push mirror via API (see docs).