Describe the bug
The v1alpha1 channelable shape has been deprecated forever and we need to remove it.
knative/eventing#5005

Looks like kafka still uses it and it's still storing channels as v1alpha1:
https://github.com/knative-sandbox/eventing-kafka/blob/master/config/channel/resources/kafkachannel-crd.yaml#L30

So, these will stop being reconciled once 5005 goes in. We should fix this asap so we can unblock the upstream work.

@matzew

A clear and concise description of what the bug is.

Expected behavior
Using supported ducks for Channelable.

To Reproduce
Steps to reproduce the behavior.

Knative release version
head

Additional context
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