[web] Update UserList to call useUserSearchIndex

Summary:
There are several search experiences in the app that do not use get/usePotentialMemberItems. This means we need to update them individually, so I'm separating out diffs to do exactly this to make it easy for reviewers. [[ https://linear.app/comm/issue/ENG-6019/[after-landing]-try-to-update-remaining-search-experiences-to-use | ENG-6019 ]] tracks attempting to update these search experiences to use usePotentialMemberItems. Each diff will affect one file ideally.

This diff affects UserList.

This component is used when:

• viewing friend list on web
• viewing block list on web

Addresses ENG-5434

Depends on D10250

Test Plan:
Please see the video below where I view the friend list and block list on web

Reviewers: atul, ginsu, inka

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10251