Deprecate and migrate KafkaChannel secret format to Broker/Sink secret format

[pierDipi]

Problem

The KafkaChannel secret format is using some different keys, as discussed in #2213, we should align it to the Broker and Sink format.
In #2224 we added support for the old format only.

Persona:
Which persona is this feature for?

• users (maintainers as well)

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?
1

Additional context (optional)

/area channel
/triage accepted

[aliok]

I thought this is working out of the box.

Did you remove the code that reads the new format with #2224 @pierDipi ?

We should fallback if for example, protocol is missing.

[pierDipi]

I didn't remove the code (it's still there because we need it for broker) but I didn't add the fallback logic