Follow Up: Autocomplete Group Users to use distinct_each_batch

What does this MR do and why?

This MR is Addressing: #431708 (closed)

A previous merged MR tried to resolve the issues on Autocomplete::UsersFinder when we search for users within a group. The new code was released behind a feature flag.

The previous MR performed badly on GPRD when it was enabled for few users, and tested with Autocomplete on gitlab-org.

This MR is to address a suggestion by @engwan, and previously by @jhyson mentioned it here by reading the user_ids from the members table distinctly instead, in an effort to reduce the number of each_batch queries that are issued to the database.

New database queries introduced in this MR.

1. First distinct_each_batch query: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/24310/commands/77671
2. Second distinct_each_batch query: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/24310/commands/77672

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #420387 (closed)