Deprecate the global `spec.high-availability` field

[matzew]

On both KnativeServing and KnativeEventing we have:

  high-availability:
    replicas: XXX

Which does a global replica setting of all deployments.apps. This was a good start in the beginning, but I think we should deprecate this, since we can do a much better fine-grain replica setting, using the spec.deployments field

[matzew]

/cc @houshengbo

[houshengbo]

@matzew We can mark it as deprecated and sterilize the functions related to this field, but will not physically remove the field(keep it for a few releases). Removal of fields in the same CRD version can lead to problems.(Ran into them before, do not want to step over again)

Once there is new CRD version, like v1, ready, then we can remove this field.

[matzew]

This is a good compromise

[xfu83]

high-availability is used by us to set minReplicas for HorizontalPodAutoscaler for knative-serving

https://github.com/knative/serving/blob/release-1.6/config/core/deployments/activator-hpa.yaml#L25

https://github.com/knative/operator/blob/8738a21c9c4f5b9d7b27fef0cc9230f264e8e2d0/pkg/reconciler/common/ha.go#L68

and i don’t think we can do the same with just the deployments.replicas https://github.com/knative/operator/blob/8738a21c9c4f5b9d7b27fef0cc9230f264e8e2d0/pkg/reconciler/common/deployments_override.go#L89

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