Use target.hidden? once the FF hide_merge_requests_from_banned_users is fully rolled out

Background

In the file ee/lib/elastic/latest/merge_request_instance_proxy.rb, we can not use data['hidden'] = target.hidden? because hidden? method on the target is guarded by a feature flag hide_merge_requests_from_banned_users. If we use data['hidden'] = target.hidden? and the feature flag hide_merge_requests_from_banned_users is disabled it will populate hidden as false for all the documents which are wrong. So we are using this data['hidden'] = target.author&.banned? to overcome this situation.

Proposal

Use data['hidden'] = target.hidden? once the feature flag hide_merge_requests_from_banned_users is fully rolled out