Include all ancestors milestones in json list & autocomplete

What does this MR do?

For project#milestones endpoint and json format it returns milestones of the relevant project and all ancestors.

The same was done in autocompletion service.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Only milestones of a project and its direct (one) parent were returned which is not expected behaviour.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #36862 (closed)