ntfy: Migrate to Kustomize

Using Kustomize, we can define the configuration file separately from
the Kubernetes resources, and use `configMapGenerators` to generate the
ConfigMap for it.  Additionally, this will make it possible to update
_ntfy_ using `updatebot`.

ntfy/kustomization.yaml

@@ -0,0 +1,23 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+namespace: ntfy
+
+resources:
+- ntfy.yaml
+
+configMapGenerator:
+- name: ntfy
+  namespace: ntfy
+  files:
+  - server.yml
+  options:
+    labels:
+      app.kubernetes.io/name: ntfy
+      app.kubernetes.io/component: ntfy
+      app.kubernetes.io/instance: ntfy
+      app.kubernetes.io/part-of: ntfy
+
+images:
+- name: docker.io/binwiederhier/ntfy
+  newTag: v2.11.0