Resolve "Group bulk edit merge requests - change milestone"

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ee/issues/12719

Allows users to bulk edit merge requests milestones at the group level.

CE port with documentation: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30569

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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