GitLab Migration - migrate Merge Request approval rules

Problem

In #369868 (closed) we added MR approvers to relations migrated with GitLab Migration, however MR approval rules are not migrated yet.

MR approvals can be set on 4 levels:

• instance level, docs
• group level, docs
• project level, API docs
• MR level, API docs

Proposed solution

Migrate MR approvals on project and MR level.