KafkaProducer operator: remove the deprecated consistent region policy "AtLeastOnce"

[ghost]

The consistent region policy defines the custom literals

• NonTransactional
• Transactional
• AtLeastOnce

AtLeastOnce is deprecated and is equivalent to NonTransactional.

The opposite of at-least-once in the sense of a better delivery could be exactly-once, but this is not in general supported with IBM Streams. Data processing in a Consistent Region has always an at-least-once guarantee. When the producer uses transactions, it is a much better at-least-once policy, but it remains at-least-once - nothing else.

[ghost]

Removing features is backward incompatible. That's why this can be done only on a major release bump.