[web] Update SubchannelMembers 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 SubchannelMembers.

This component is used when adding subchannel members during creation on web

Addresses ENG-5434

Depends on D10249

Test Plan:
Please see the video below where I show the modal when adding subchannel members

Reviewers: atul, ginsu, inka

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10250