Investigate and reduce N+1 SQL issues on notes GraphQL field
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## Background
Based on investigation done on https://gitlab.com/groups/gitlab-org/-/epics/12441#note_1730085427, the `notes` field in `GetProjectMergeRequests` query can results to N+1 SQL queries.
## Proposal
Investigate and reduce the N+1 SQL queries when `notes` is queried.
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