[terraform] return to 2 identical identity service instances

Summary:
We've removed the in-memory cache from the identity service, so it's safe to scale back up to 2 nodes

Depends on D11257

Test Plan: Tested this in D11257 on staging

Reviewers: will, ashoat, bartek

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11260