grafana: setup sendAlert = false receiver to mute alert

resources/chaosknoten/grafana/docker_compose/alertmanager.yaml.j2

@@ -9,6 +9,9 @@ route:
   group_interval: 5m
   repeat_interval: 6h
   routes:
+    - receiver: "null"
+      matchers:
+        - sendAlert = "false"
     - receiver: ntfy-ccchh-critical
       matchers:
         - org = "ccchh"
@@ -48,6 +51,7 @@ templates:
   - "/etc/alertmanager/templates/*.tmpl"
 
 receivers:
+  - name: "null"
   - name: "ccchh-infrastructure-alerts"
     telegram_configs:
       - send_resolved: true