Migrate fanout message handler to duck v1

[aliok]

Problem
Fanout message handler uses duck v1beta1 api and because of that we can't migrate the KafkaChannel to use v1 duck api.

eventing/pkg/channel/fanout/fanout_message_handler.go

Line 39 in 15f4c11

eventingduck "knative.dev/eventing/pkg/apis/duck/v1beta1"

Persona:
Which persona is this feature for?

Exit Criteria
A measurable (binary) test that would indicate that the problem has been resolved.

Time Estimate (optional):
How many developer-days do you think this may take to resolve?

Additional context (optional)
knative/eventing-contrib#1395

[slinkydeveloper]

/assign