Promote KafkaChannel to v1

[matzew]

Problem
KafkaChannel should be promoted to v1. Would be best as a follow up on after the refactorings and unifications:

• [meta] KafkaChannel refactorings #386
• [meta] KafkaChannel unification #387

Persona:
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)
Add any other context about the feature request here.

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[aliok]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[pierDipi]

/reopen
/remove-lifecycle stale

[knative-prow-robot]

@pierDipi: Reopened this issue.

In response to this:

/reopen
/remove-lifecycle stale

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[pierDipi]

/triage accepted