[terraform] Bump prod Identity to 0.51.1

Summary:
This version has been on staging for 3 days now, it also has been tested on prod during live debugging session and looks stable now.
Changes described in D14392.

Depends on D14392

Test Plan: terraform plan

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14394