API: Use Search::GlobalService in projects search API

Originally opened at !6252 (closed) by @Razzeee.

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What does this MR do?

It uses a proper service to search for projects in the GET /projects/search/:query API endpoint.

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

No, but this endpoint should probably be removed entirely in the future, as it's possible to search for project with GET /projects?search=hello (see #22007 (closed)).

Why was this MR needed?

The idea was to have a lowercase search.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• API support added
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together