[identity] Return use_v1_flow in new-flow-only RPCs

Summary:
Address ENG-9695

Depends on D14074

Test Plan: Registered a user (without backup), then tried to call e.g. primary device logout RPC. The use_v1_flow was returned in response.

Reviewers: kamil, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D14075