Replace the 'project/milestone.feature' spinach test with an rspec analog

What does this MR do?

This MR moves all the test from the features/project/milestone.feature spinach spec to an rspec analog.

Why was this MR needed?

This is a part of the spinach removal campaign.

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

What are the relevant issue numbers?

#23036 (closed)