fix: move watchdog to null rec

Author: henrywhitaker3

argo/cluster/monitoring/kube-prometheus-stack/chart/alertmanager/config.yaml

@@ -10,7 +10,7 @@ route:
     - alertname
 
   routes:
-    - receiver: heartbeat
+    - receiver: "null"
       matchers:
         - alertname=~"Watchdog"
       group_wait: 0s