V1beta1 reconcile triggers mt

[vaikas]

Fixes #

Proposed Changes

• Reconcile Triggers using v1beta1.

Release Note

- 🧽 Update or clean up current behavior
Reconcile Triggers as v1beta1 instead of v1alpha1.

Docs

[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/trigger_lifecycle.go	51.1%	50.0%	-1.1
pkg/reconciler/broker/broker.go	81.2%	82.4%	1.3
pkg/reconciler/broker/trigger.go	87.0%	86.9%	-0.1
pkg/reconciler/mtbroker/broker.go	73.0%	73.2%	0.2
pkg/reconciler/mtbroker/trigger.go	87.7%	87.6%	-0.1

[n3wscott]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[knative-prow-robot]

@vaikas: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-knative-eventing-build-tests	fe79477	link	/test pull-knative-eventing-build-tests

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