Replace Traefik with kamal-proxy

[kamal-proxy](https://github.com/basecamp/kamal-proxy) is a custom
minimal proxy designed specifically for Kamal.

It has some advantages over Traefik:
1. Imperative deployments - we tell it to switch from container A to
   container B, and it waits for container B to start then switches. No
   need to poll for health checks ourselves or mess around with forcing
   health checks to fail.
2. Support for multiple apps - as much as possible, configuration is
   supplied at runtime by the deploy command, allowing us to have
   multiple apps share a proxy without conflicting config.
3. First class support for Kamal operations - rather than trying to
   work out how to make Traefik do what we want, we can build features
   directly into the proxy, making configuration simpler and avoiding
   obscure errors

test/integration/docker/deployer/setup.sh

@@ -19,7 +19,7 @@ push_image_to_registry_4443() {
 
 install_kamal
 push_image_to_registry_4443 nginx 1-alpine-slim
-push_image_to_registry_4443 traefik v2.10
+push_image_to_registry_4443 basecamp/kamal-proxy latest
 push_image_to_registry_4443 busybox 1.36.0
 
 # .ssh is on a shared volume that persists between runs. Clean it up as the