Alertmanager failed due to unsupported scheme URL used in receiver in RHOCP 4.

Issue

• Alertmanager pods are in CrashLoopBackOff :

  $ oc get pods -n openshift-monitoring -l app=alertmanager
  
  alertmanager-main-0                                     5/6     Error              24         1h
  alertmanager-main-1                                     5/6     CrashLoopBackOff   21         1h
  

• Alertmanager pods fail due to unsupported scheme that is used in URL :

  $ oc logs alertmanager-main-1 -n openshift-monitoring -c alertmanager | grep -i "unsupported scheme"
  
  2025-06-05T13:38:47.752037727+08:00 ts=2025-06-05T05:38:47.752Z caller=main.go:348 level=warn msg="unable to join gossip mesh" err="2 errors occurred:\n\t* Failed to resolve alertmanager-main-0.alertmanager-operated:9094: lookup alertmanager-main-0.alertmanager-operated on 172.00.0.10:53: no such host\n\t* Failed to resolve alertmanager-main-1.alertmanager-operated:9094: lookup alertmanager-main-1.alertmanager-operated on 172.13.0.10:53: no such host\n\n"
  2025-06-05T13:38:47.785152302+08:00 ts=2025-06-05T05:38:47.785Z caller=coordinator.go:118 level=error component=configuration msg="Loading configuration file failed" file=/etc/alertmanager/config_out/alertmanager.env.yaml err="unsupported scheme \"\" for URL"