Description of problem: After enabling the Observability, the below alert forward configuration is not present in the configmap cluster-monitoring-config on the ROSA managed cluster

```

prometheusK8s:
      additionalAlertmanagerConfigs:
      - apiVersion: v2
        bearerToken:
          key: token
          name: observability-alertmanager-accessor
        scheme: https
        staticConfigs:
        - alertmanager-open-cluster-management-observability.apps.cqu-obs-delta.dev09.red-chesterfield.com
        tlsConfig:
          ca:
            key: service-ca.crt
            name: hub-alertmanager-router-ca
          insecureSkipVerify: false
      externalLabels:
        managed_cluster: c13738b2-3825-46f0-966d-7126ca50c246

```

Version-Release number of selected component (if applicable): 2.12 DS build

How reproducible:

Steps to Reproduce:

1. import ROSA cluster into the Hub
2. Enable the Observability
3. check the configmap cluster-monitoring-config, there's no alert forward configuration added on the ROSA cluster.

Actual results:

Expected results:

Additional info: