Refactor group search out of global search

What does this MR do?

Add Search::GroupService and uses it instead of Search::GlobalService for group search

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

Why was this MR needed?

Not necessarily in CE, but certainly in EE, this logic was getting too complicated. See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1649

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests
  • 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 #30615 (closed)