[terraform] Make Tunnelbroker deployments more future-proof

Summary:
Tunnelbroker deployments on prod sometimes failed due to The target group tunnelbroker-grpc-tg does not have an associated load balancer.
This was due to disabled LB listener for gRPC, so the target group had no listeners.
Detaching the ECS Service from adding Tunnelbroker to this blind target group resolved the issue.
Added appropriate comments

Test Plan: Terraform apply succeeded on prod after previously failing with the message. Opened AWS Console and made sure Tunnelbroker is not registered to the gRPC target group.

Reviewers: will, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13360