Use autocomplete query in assignees widget to avoid duplicates

What does this MR do and why?

This MR uses the projectAutocompleteUsers query in assignees widget , i.e the same that is used in legacy issues to avoid duplicates in assignees widget. This also removes the infinite scroll since that is not supported in the new query

Please read this thread for all the context

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Screenshots or screen recordings

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Group level work item assignees	Project level work item assignees
Screen_Recording_2024-02-22_at_6.06.17_PM	Screen_Recording_2024-02-22_at_6.07.11_PM

How to set up and validate locally

1. In rails enable the FF

   Feature.enable(:work_items_mvc_2)

2. Visit any group or project level work item , see that you see no duplicates and correct assignees as in legacy issues/epics

P.S : I could not replicate the duplicate users issue in my local environment