Consider deprecating control plane machines outside a control-plane provider

[vincepri]

In v1alpha4, we should consider dropping support for single control plane machines, instead making sure users adopt a control plane providers.

/area api
/kind cleanup
/milestone v0.4.0

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/lifecycle stale

[ncdc]

/remove-lifecycle stale

[vincepri]

/kind api-change
/lifecycle frozen

[fabriziopandini]

+100

[vincepri]

@fabriziopandini This is something to consider for CABPK, although not sure how we'd go about enforcing it.

[CecileRobertMichon]

/kind documentation

let's document best practice in v1alpha4 + warn users if no control plane ref exists in the cluster.
In vNext we should enforce that all control plane machines are owned by a control plane object (eg. KCP).