Don't use Elasticsearch join for group/project MR searches

What does this MR do?

When searching for merge requests in the context of a group we don't need to use an Elasticsearch join. The join logic defined in ApplicationClassProxy is not necessary as we are already filtering down based on the project ID list the user has access to and hence we can just use the target_project_id field instead of joining to the project and using the ID field.

This MR is basically identical to !46461 (merged) but in this case we need to use target_project_id instead of project_id since the MR only had target_project_id until recently and project_id is not yet backfilled.

This change on it's own will make searches more performance (since this term query is more efficient than a join) but it will additionally be the next step in allowing us to move merge requests to a separate index in Elasticsearch &5468 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Separation of EE specific content

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Related to #322784 (closed)