Rename bind DNS namespaces to ns-* (#223)

Renames the three BIND DNS app namespaces `binddns-{auth,resolver,externaldns}` -> `ns-{auth,resolver,externaldns}`.

## Why
Shorter, clearer namespace names for the DNS tiers.

## Changes
- `argocd/applicationsets/platform.yaml`: overlay path registrations renamed (the ApplicationSet derives each app's namespace from its overlay dir name)
- `argocd/projects/platform.yaml`: destination namespaces renamed

## Coupled with
The per-tier PRs (#220/#221/#222) rename the overlay dirs + namespaces + external-dns hostnames to match. No app deploys to a renamed namespace until both this and the tier PR are merged (harmless before then — the ApplicationSet only instantiates apps for existing dirs).

Reviewed-on: #223
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

argocd/projects/platform.yaml

@@ -21,11 +21,11 @@ spec:
       server: https://kubernetes.default.svc
     - namespace: 'authentik'
       server: https://kubernetes.default.svc
-    - namespace: 'binddns-auth'
+    - namespace: 'ns-auth'
       server: https://kubernetes.default.svc
-    - namespace: 'binddns-resolver'
+    - namespace: 'ns-resolver'
       server: https://kubernetes.default.svc
-    - namespace: 'binddns-externaldns'
+    - namespace: 'ns-externaldns'
       server: https://kubernetes.default.svc
     - namespace: 'cert-manager'
       server: https://kubernetes.default.svc