Fix N+1 when indexing data

What does this MR do and why?

Related to #426110 (closed) An N+1 was observed when debugging a recent incident when indexing data for search into Elasticsearch.

This MR:

• adds a missing N+1 test for milestones, fixes the N+1 discovered
• slight change to existing epic N+1 test, fixes the N+1 discovered
• create new preload scopes to avoid rubocop disable

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

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