Deprecation of prometheus-adapter

From https://github.com/kubernetes-sigs/prometheus-adapter/issues/701

> We want to deprecate the project in the near-future, post-triage, and suggest users to fulfill their use-cases using KEDA.

When the deprecation materializes, kube-prometheus should probably stop installing prometheus-adapter by default.

Related to https://github.com/prometheus-operator/kube-prometheus/issues/2475