[lib] use v2 relevant followers API to find farcaster friends

Summary: this API returns what we want -- farcaster mutuals. and it seems to work pretty consistently, unlike the paginated v1 followers API.

Test Plan: successfully got back farcaster mutuals for dwr.eth (fid 3), who has a ton of followers

Reviewers: ashoat, will

Reviewed By: ashoat, will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13530