fix: Remove tls:external from Rancher HelmRelease

With Tailscale LoadBalancer, TLS is not actually terminated at the edge.
The Tailscale proxy does TCP passthrough, so Rancher must serve its own
TLS certs. Setting tls: external caused Rancher to listen HTTP-only,
which broke HTTPS access through Tailscale.

infrastructure/addons/rancher/helmrelease-rancher.yaml

@@ -23,7 +23,6 @@ spec:
       retries: 3
   values:
     hostname: rancher.silverside-gopher.ts.net
-    tls: external
     replicas: 1
     extraEnv:
       - name: CATTLE_PROMETHEUS_METRICS