Tidy up `spec/requests/api/issues/put_projects_issues_spec.rb`

Specifically attempting to remove the huge duplication of api("/projects/#{project.id}/issues/#{issue.iid}", user)

Perhaps in a similar way to !31522 (diffs);

let(:api_base) { api("/projects/#{project.id}/merge_requests/#{merge_request.iid}", user) }

The following discussion from !31864 (merged) should be addressed:

• @leetickett started a discussion: (+3 comments)

  @rpaik hackathon MR

  @robotmay @igor.drozdov this is a similar MR to !31522 (merged) but for issues rather than merge requests. Would you mind taking a look?

  One of the points raised previously related to repetition (DRY) of the PUT API URL. As I had newly introduced this to the MR spec I made the change, but in this instance I wonder whether I should raise a separate MR to refactor the already considerable duplicate code?

  Let me know what you think- thanks!

And separating set-up and expectations with a newline as per https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#general-guidelines