[terraform] Use subdomain for staging services

Summary:

This stack resolves issues encountered when setting up fresh staging AWS account with Terraform.

I've manually set up a separate Route53 hosted zone for the staging subdomain and appropriate SSL certs. Steps are described in the "Non-Terraform AWS setup" Notion doc.

This diff adjusts terraform to use the new subdomain for staging services.

Depends on D8712

Test Plan: Terraform plan for production gives no changes. Terraform plan for staging shows changes to the subdomain.

Reviewers: jon, varun

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8713