[terraform] bump identity service to 0.6

Summary: i've retagged 0.6-staging as 0.6 on docker hub. will run terraform apply on both staging and prod when i land this diff.

Test Plan: confirmed that the new image is good by calling the DeleteUser RPC which was recently moved to the auth proto.

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10354