Add a separate target for basic merge request search SLI
What does this MR do and why?
Basic merge request searches that are scoped to a group are running for longer than the other non-code searches. This negatively impacts the global search error budget. It is known that the merge request table takes longer to run database queries and the ideal is to move all merge request searches from basic (database) to advanced (Elasticsearch).
This MR adds a separate target for basic merge request search that is higher than the other non-code searches so that we can temporarily fix the error budget and look into either fixing the queries or moving to advanced search sooner.
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Related to https://gitlab.com/gitlab-org/search-team/team-tasks/-/issues/164