docs: Update changelog for dropped cAdvisor metrics

Author: philipgough

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - [#1241](https://github.com/openshift/cluster-monitoring-operator/pull/1241) Add config option to disable Grafana deployment.
 - [#1278](https://github.com/openshift/cluster-monitoring-operator/pull/1278) Add EnforcedTargetLimit option for user-workload Prometheus.
+- [#1291](https://github.com/openshift/cluster-monitoring-operator/pull/1291) Drop high caredinality cAdvisor metrics via [kube-prometheus #1250](https://github.com/prometheus-operator/kube-prometheus/pull/1250)
 - [#1270](https://github.com/openshift/cluster-monitoring-operator/pull/1270) Show a message in the degraded condition when Platform Monitoring Prometheus runs without persistent storage. 
 
 ## 4.8