[keyserver] Rework updateRelationships on keyserver to use new API

Summary:
This diff updates updateRelationships to take the new API as input, and adds code for converting the old input into the new format to legacyUpdateRelationshipsResponder

Depends on D13437

Test Plan: I tested the API as part of my work on ENG-9331

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13438