[identity] Better handle new flow users in v1 logout

Summary:
Addresses 1 and 2a from ENG-10202.

• For v2 flow primary device logout, display an error message
• For v1 flow primary device logout, remove backup to future-proof restore flow (make sure it falls back to v1).

Test Plan: Tested on staging

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14380