[terraform] update staging identity server image to 0.16-staging

Summary: This updates our staging identity server image to latest version of 0.16-staging

Test Plan: Successfully deployed on AWS and ensured that I was getting responses from rpcs and websocket server

Reviewers: ashoat, bartek

Reviewed By: bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11977