Draft: Resolve "Follow-up from "Resolve "Add command to delete staged projects push mirrors"" - refactor pull mirroring feature"

Overview

The goal of this MR is to refactor the pull mirroring commands.

This is so we can still offer the feature for large customers that cannot upgrade their GitLab instance and cannot use Project export/import due to:

1. Low GitLab version that doesn't have the Project export/import API (<10.6)
2. Low GitLab version that does not have an export/import version compatible with the latest (0.2.4, as of 11.1)

Tasks

• Update docs if needed (workflow, runbooks, README, migration metrics, etc.)

Acceptance Criteria

• Existing UTs pass locally

@gitlab-org/professional-services-automation/tools/migration

Closes #790