[terraform] temporarily reduce identity service desired count

Summary: grpc-web seems to be ignoring the load balancer cookie, causing identity login to fail sporadically. while we investigate possible solutions, we can temporarily reduce the number of identity nodes to 1 to unblock @inka

Test Plan: confirmed that identity login succeeds 10 times in a row after reducing the number of nodes. previously, it was failing roughly 4/5 attempts

Reviewers: ashoat, will

Reviewed By: ashoat, will

Subscribers: tomek, inka

Differential Revision: https://phab.comm.dev/D11138