Replace the `project/issues/milestones.feature` spinach test with an rspec analog

What does this MR do?

1. Moves all the tests from the features/issues/milestones.feature spinach spec to an rspec analog.
2. Refactors spec/features/milestones/show_spec.rb

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

No.

Why was this MR needed?

This is a part of the spinach removal campaign.

Screenshots (if relevant)

No.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#23036 (closed)