feat: move namespaces defs into apps and utilize fluxcd.io/prune label

Signed-off-by: Devin Buhl <devin@buhl.casa>

cluster/apps/kustomization.yaml

@@ -2,7 +2,9 @@
 apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 resources:
-  # - system-upgrade
   - default
+  - flux-system
   - kube-system
+  - metallb-system
   - networking
+  # - system-upgrade