Add service to transfer Group Milestones when transferring a Project

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ce/issues/60372

This issue points out that after transferring a project, its issuables can retain the previous group milestone, and this will prevent them from being edited by throwing an invalid milestone error.

Currently, we are using Labels::TransferService to transfer group labels by creating a replica at the transferred project level. This MR implements a similar service that will replicate group milestones (also at the project level) and assign it to the corresponding issuables.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers