Description of problem:

As per https://prometheus.io/docs/alerting/latest/alertmanager/#high-availability, when configuring Alertmanager HA, the Prometheus should send requests to all alertmanagers.

RHACM run 3 alertmanagers in clustered mode in the Hub cluster. However we send alerts over a single Route from the managed clusters configuration which means we might run into the problems described in https://groups.google.com/g/prometheus-users/c/ul3tFfPxkC0 

Discussed in https://redhat-internal.slack.com/archives/CUU609ZQC/p1723548761663429

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