Promote HeadroomConfig API to v1beta1

Event: the spec goes a full release cycle with no field changes and Q21 (Helm) has shipped — or Demand: an external consumer needs a supported (non-alpha) API.

The Event fired with v0.2.0. Q21 shipped before v0.1.0, and git diff v0.1.0 v0.2.0 -- api/ config/crd/bases changes one line, the controller-gen.kubebuilder.io/version annotation (v0.21.0 to v0.23.0). No field changed.