Resolve "Populate the milestone dropdown combobox on the Release edit/new page with Group milestones"

What does this MR do?

This MR resolves #232577 (closed) by adding Group Milestones to the milestone combobox on new/edit Release pages.

It allows users to search for group milestones, and displays them in a group milestones section under the project milestones section.

The user can then select group milestones (along with project milestones) to associate with a release.

Screenshots (strongly suggested)

When not on GitLab Premium or the project doesn't belong to the group, the group milestones feature does not render:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #232577 (closed)