[lib] Rename UPDATE_RELATIONSHIP message type to LEGACY_UPDATE...

Summary:
We're going to create a new version of UPDATE_RELATIONSHIP which can support the farcaster_mutual action.

Depends on D11381

Test Plan: Straightforward rename. flow + CI

Reviewers: ashoat, ginsu, varun, will

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11382