Improve MilestonesFinder to accept project and group relations

What does this MR do?

!23098 (merged) fixed the scope Milestone.for_projects_and_groups. It now correctly handles arrays of ids / objects and relations.

This reverts !19863 (merged) and !19920 (merged) and also fixes MilestonesFinder which used to force relations into an array of AR objects.

What are the relevant issue numbers?

Closes #47988 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer