gitlab-com-infrastructure should leverage GitLab Managed Terraform state backend

Created based on https://ops.gitlab.net/gitlab-com/gitlab-com-infrastructure/-/issues/6 that was opened by @nicholasklick under the gitlab-com-infrastructure project tracker on ops.

In an effort to dogfood the GitLab managed Terraform state backend, Let's advocate that this project attempt to utilize it as well!

Goal

gitlab-com-infrastructure should use GitLab managed Terraform state backend

Definition of done

• Replace all backend references (example) with http
• Change terraform init (example) to invoke the correct set of config params outlined here. Example
• Change any relevant documentation