feat: flatten core directory

cluster/core/namespaces/kustomization.yaml

@@ -2,4 +2,4 @@ apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 resources:
 - cert-manager.yaml
-- networking.yaml
+- metallb-system.yaml

cluster/core/namespaces/metallb-system.yaml

@@ -2,4 +2,4 @@
 apiVersion: v1
 kind: Namespace
 metadata:
-  name: networking
+  name: metallb-system