v-m/alertmanager: Suppress battery low alerts

The alerts for Z-Wave device batteries in particular are pretty
annoying, as they tend to "flap" for some reason.  I like having the
alerts show up on Alertmanager/Grafana dashboards, but I don't
necessarily need notifications about them.  Fortunately, we can create a
special "none" receiver and route notifications there, which does
exactly what we want here.

victoria-metrics/alertmanager.config.yml

@@ -11,12 +11,16 @@ receivers:
 - name: ntfy
   webhook_configs:
   - url: http://alertmanager-ntfy:8000/hook
+- name: none
 
 route:
   group_by:
   - '...'
   receiver: ntfy
   routes:
+  - receiver: none
+    matchers:
+    - alertname=Battery Low
   - receiver: ntfy
     matchers:
     - alertname=DiskUsage