[terraform] bump production identity service to 0.28

Summary: 0.28-staging has been in use in our staging environment for over a week. it resolves some issues like the reserved wallet registration workflow, so it would be beneficial to get this into production now.

Test Plan: terraform plan

Reviewers: will, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12705