Support Group Milestones to be associated with Project Releases in API

What does this MR do?

As a breakdown of Support release association to group milestones (Edit release page), we'll need to modify the releases API to allow for submissions that GET, POST, UPDATE, and DELETE group milestones to be associated with a release.

Group milestones that belong to the project's immediate group can be added to the existing milestones array. There is existing validation in place that ensures there is not a naming clash between project and project's group milestones.

The signature if the of the release API call does not hange, and the groups could be added to the same array, ie:

curl --header 'Content-Type: application/json' --request PUT --data '{"name": "milestone release", "milestones": ["release-milestone-1","subgroup-milestone"]}' --header "PRIVATE-TOKEN: <redacted>" "http://127.0.0.1:3000/api/v4/projects/20/releases/v25" | jq

Where release-milestone-1 is the project milestone, and subgroup-milestone is the group milestone.

Screenshots

Does this MR meet the acceptance criteria?

Additional information

More information about prepending EE code onto ActiveSupport::Concern:

• !7079 (merged)
• gitlab-foss#50824 (closed)
• !7202 (diffs)
• #23932 (closed)

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

Part of #235391 (closed)