Audit and validate the categories for CRDs

[vaikas]

Describe the bug
Some of the CRD categories appear to be wrong (at least subscription shows up under eventing) vs. (channels, imc). We should make sure we audit and validate them.

https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#categories

Expected behavior
If I ask for messaging components, subscription for example should be there.
A clear and concise description of what you expected to happen.

To Reproduce
assuming you have some resources. If you do

vaikas-a01:eventing vaikas$ kubectl get eventing
NAME                                           READY   REASON                               BROKER    SUBSCRIBER_URI                                      AGE
trigger.eventing.knative.dev/goodbye-display   True                                         default   http://goodbye-display.default.svc.cluster.local/   2d
trigger.eventing.knative.dev/mytesttrigger     False   Unable to get the Subscriber's URI   default                                                       2d2h
trigger.eventing.knative.dev/mytesttrigger2    False   Unable to get the Subscriber's URI   default                                                       2d2h

NAME                                       READY   REASON   URL                                                                             AGE
broker.eventing.knative.dev/default        True             http://broker-ingress.knative-eventing.svc.cluster.local/default/default        4d22h
broker.eventing.knative.dev/default-bare   True             http://broker-ingress.knative-eventing.svc.cluster.local/default/default-bare   20h

NAME                                                                                              READY   REASON   AGE
subscription.messaging.knative.dev/default-goodbye-display-a3424f95-d471-4e04-9a1e-fcb95e4b29df   True             2d
subscription.messaging.knative.dev/with-dead-letter-sink                                          True             7d16h
subscription.messaging.knative.dev/with-dead-letter-sink-2                                        True             7d16h

Note the subscription shows up there.

vaikas-a01:eventing vaikas$ kubectl get messaging
NAME                                       READY   REASON   URL                                                      AGE
channel.messaging.knative.dev/my-channel   True             http://my-channel-kn-channel.default.svc.cluster.local   7d16h

NAME                                                             READY   REASON   URL                                                                    AGE
inmemorychannel.messaging.knative.dev/default-bare-kne-trigger   True             http://default-bare-kne-trigger-kn-channel.default.svc.cluster.local   20h
inmemorychannel.messaging.knative.dev/default-kne-trigger        True             http://default-kne-trigger-kn-channel.default.svc.cluster.local        4d22h
inmemorychannel.messaging.knative.dev/my-channel                 True             http://my-channel-kn-channel.default.svc.cluster.local                 7d16h
inmemorychannel.messaging.knative.dev/my-channel-2               True             http://my-channel-2-kn-channel.default.svc.cluster.local               7d16h

But not there.

The one question is the expectation here. If I do:
kubectl get eventing, should that only return eventing.knative.dev resources? Or should eventing return everything from [eventing,flows,messaging].knative.dev? I'm thinking the former.

Knative release version
v0.16

Additional context
Add any other context about the problem here such as proposed priority

[MIBc]

I can help.