Alertmanager fails due to unmarshal errors

Issue

• Alertmanager pods keep restarting.

  $ oc get pods -n openshift-monitoring | grep alert
  NAME                                                    READY  STATUS   RESTARTS  AGE
  alertmanager-main-0                                     5/6    Running  6         8m40s
  alertmanager-main-1                                     5/6    Running  6         8m40s
  

• Alertmanager pods fail due to unmarshal errors:

  ts=2023-11-02T14:41:52.336Z caller=coordinator.go:118 level=error component=configuration msg="Loading 
  configuration file failed" file=/etc/alertmanager/config_out/alertmanager.env.yaml err="yaml: unmarshal 
  errors:\n line 44: cannot unmarshal !!str `severit...` into []string"