Migrate Group Milestones 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](https://gitlab.com/groups/gitlab-org/-/epics/4619) with Group Export/Import, this issue adds [Group Milestones](https://docs.gitlab.com/ee/user/project/milestones/#milestones) to Group Migration.
```graphql
query groupMilestones($full_path: ID!, $cursor: String) {
group(fullPath: $full_path) {
milestones(first: 100, after: $cursor, includeDescendants: false) {
pageInfo {
next_page: endCursor
has_next_page: hasNextPage
}
nodes {
title
description
state
due_date: dueDate
created_at: createdAt
updated_at: updatedAt
}
}
}
}
```
### 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
<!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION -->
*This page may contain information related to upcoming products, features and functionality.
It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes.
Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.*
<!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION -->
issue