Upgrade to class based broker controller

[liu-cong]

Describe the bug
The broker controller now only reconciles brokers with ChannelBasedBroker annotation. And we have defaulting and conversion webhooks to set the default broker class.

However, for brokers created before the new controller/webhooks, our new controller will ignore them because they don't have the annotation. If you do kubectl get, you will still see the annotation set by the conversion webhook. However, this annotation is not persisted for brokers created before the new webhooks.

Expected behavior
Previously created brokers should get reconciled after upgrading to the new broker controller.

To Reproduce

• Install a previous release of eventing, create a broker, verify that the broker doesn't have the brokerclass annotation.
• Install broker controller and webhooks from master
• Create a trigger to the previously created broker, and the broker is not reconciled.

Additional context
I can think of the following options:

1. Ask the user to manually modify the brokers so the defaulting webhook will add the annotation. Bad UX.
2. Have an upgrade job to list all brokers without brokerclass and set the default broker class.
3. Have the broker controller reconcile brokers with no brokerclass annotation, POC here Allow default broker controller to reconcile unset brokerclass. #2761