observability-bundle release v2.9.0

Added

• Add Backstage audience annotations.
• Add managementCluster: "" as a top-level value (populated from the cluster chart via defaultValues)
• Moves full KSM metricRelabelings ownership from kube-prometheus-stack-app into observability-bundle

Changed

• Update dependency kube-prometheus-stack-app and prometheus-operator-crd to v21.0.0
• Update alloy-app to 0.19.0