Resolve "Use GraphQL `autocompleteUsers` in assignees widget"

What does this MR do and why?

This MR replaces current projectMembers query on user_select with autocompleteUsers query

Screenshots or screen recordings

No visual changes

How to set up and validate locally

1. In rails console enable the GraphQL assignees widget FF

   Feature.enable(:issue_assignees_widget)

2. Open any issue and try searching for users on the sidebar -> Assignees
3. Repeat the same step for MR sidebar and for issue boards sidebar

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #419402 (closed)