[web] Update MembersModal 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 MembersModal.

This component is used when viewing channel members on web.

Addresses ENG-5434

Depends on D10231

Test Plan:
Please see the video below where I view the channel members on web

Reviewers: atul, ginsu, inka

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10249