Use `simple=true` for projects API in Projects dropdown for better search performance

What does this MR do?

Now that projects API exposes avatar_url even when simple=true is used as query param (with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14498), we could use it for Projects dropdown and make search results within projects dropdown faster.

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?

Closes #38052 (closed)