Add validation for migrated mr reviewers

What does this MR do and why?

Part of: #368100 (closed)

How to set up and validate locally

bundle exec rspec qa/specs/features/api/1_manage/migration/gitlab_migration_mr_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.