Remove N+1 DB queries from indexing issues in Elasticsearch

What does this MR do?

The #as_indexed_json method is used to convert individual documents to the JSON that is sent to Elasticsearch during indexing. We need to preload the data before it gets to this method. We implemented the .preload_indexing_data method in !56808 (merged) so we just need to add this to the various classes now. This MR updates it for Issue only.

We also slightly refactor the issue_instance_proxy since it's more efficient to just use the user_id from the issue_assignees join table than to join again to users and also since it wasn't possible to get the preload to work without loading all of users.* which would be wasteful for a wide table and large numbers of records.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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Related to #324745 (closed)