Reconcile MT Broker using v1beta1 + fallout

[vaikas]

Fixes #

Proposed Changes

• Reconcile Broker for MT ChannelBasedBroker using v1beta1 API.

Release Note

- 🧽 Update or clean up current behavior
Reconcile Brokers using v1beta1 API

Docs

[vaikas]

Just a note about these tests, I removed locally the eventing/v1alpha1 directory to make sure I got all the references to them. Mainly a note to myself...

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/eventing/v1beta1/broker_lifecycle.go	83.3%	95.5%	12.1
pkg/apis/eventing/v1beta1/test_helper.go	62.5%	68.6%	6.1
pkg/mtbroker/filter/filter_handler.go	78.3%	77.8%	-0.5
pkg/reconciler/mtbroker/broker.go	73.2%	85.4%	12.3
pkg/reconciler/mtbroker/trigger.go	87.6%	87.5%	-0.1
pkg/reconciler/source/duck/duck.go	71.0%	70.7%	-0.3

[matzew]

/assign @matzew

[n3wscott]

/lgtm
/approve

[matzew]

/lgtm
/approve

also tested w/ e2e sample on k8s 1.18.3

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: matzew, n3wscott, vaikas

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