[terraform] Bump prod Tunnelbroker to 0.18.3

Summary:
Deploying D14254

Tunnelbroker images are [here](https://hub.docker.com/r/commapp/tunnelbroker/tags)

[tunnelbroker-v0.18.3](https://github.com/CommE2E/comm/tree/tunnelbroker-v0.18.3)

This image was for a week on a staging now - no issues found.

Test Plan: terraform plan

Reviewers: tomek, angelika

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14292

services/terraform/remote/service_tunnelbroker.tf

@@ -1,7 +1,7 @@
 locals {
   tunnelbroker_config = {
     docker_image = "commapp/tunnelbroker"
-    docker_tag   = local.is_staging ? "0.18.3" : "0.18.2"
+    docker_tag   = local.is_staging ? "0.18.3" : "0.18.3"
 
     grpc_port      = 50051
     websocket_port = 51001