Refactor dropdown_assignee_spec

What does this MR do?

Refactoring dropdown_assignee_spec using capybara methods for waiting content instead of using sleep or wait_for_ajax methods

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

I had random rare failure in "filters by" group of tests when testing multiple times locally. I increased timeout capybara setting but could find any correlation between those.

Why was this MR needed?

To decrease significantly execution time of the tests.

Does this MR meet the acceptance criteria?

• Changelog entry added
• [n/a] Documentation created/updated
• API support added
• Tests
  • [n/a] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27568 (closed)