Migrate Iterations with the Group

Problem to solve

For the Group Migration to be a viable solution for the users migrating Groups, we need to achieve parity with the Group Export/Import feature.

Proposed Solution

As part of the larger epic targeting parity with Group Export/Import, this issue adds Iterations to Group Migration.

Documentation

Following documentation should be updated with the fields added in this issue:

• User docs: https://docs.gitlab.com/ee/user/group/import/
• Dev docs: https://docs.gitlab.com/ee/development/bulk_import.html