[keyserver] return default avatar to old clients when avatar type is "farcaster"

Summary:
when a viewer tries to fetch their own avatar or another user's avatar on an old client, we should return the default avatar. older clients will not be able to handle farcaster avatars

Depends on D13378

Test Plan: default avatar displays on older native app

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D13379