[lib] Remove single usage of LegacyMemberInfo

Summary:
Part of https://linear.app/comm/issue/ENG-6366/rename-minimallyencodedinfo-to-rawinfo

Unfortunately this is the only usage of LegacyMemberInfo that we're able to remove.

Depends on D10789

Test Plan: flow

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10790