Prometheus externalLabels `Cluster:` enforced from RHOCP 4.20

Issue

• The protected label cluster can no longer be used as an externalLabel in Prometheus from Openshift 4.20
• Using the cluster key name can cause issues with developer dashboards

• In the events from the openshift-monitoring namespace could be present the error:

  error unmarshaling: reserved key "cluster" (one of [prometheus prometheus_replica cluster]) cannot be set in externalLabels