WIP: Add subepics checks into services and api

What does this MR do?

This is a second iteration towards enabling epics without epic children for premium.

This MR adds checks to epic link services, if subepics feature is not available (not on ultimate) an error is returned (update, create) or an empty list of children (list).

It also adds checks to the API and graphQL (TODO).

The last iteration will be to move epic to premium in license.rb and integrating with frontend

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.

Related issue: #37081 (closed)