Resolve "Follow-up from "Milestone tests""

What does this MR do?

This MR extracts the has_milestone? method out of QA::Page::MergeRequest::Show and into QA::Page::Issuable::Sidebar.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48690 (closed)