Deprecate ZooKeeper-based cluster fields in the API

[scholzj]

Type of change

• Task

Description

This PR deprecates the API fields related to ZooKeeper-based clusters:

• Deprecated .spec.zookeeper section
• Deprecated .spec.kafka.replicas section
• Deprecated .spec.kafka.storage section
• Unused ZooKeeper-related methods from KafkaResources were removed
• The tests related to the Kafka CR were cleaned up to focus on Kraft-based clusters and maintain just one legacy ZooKeeper-based Kafka CR YAML to make sure we still support it in the CRDs
• As we now have the automatic deprecation warnings, the warnings generated from KRaftUtils were removed as they are not needed anymore
• In System Tests, some of these deprecated fields are still used and this will be cleaned only when ZooKeeper is cleaned-up from the STs. So for the time being, the deprecation warnings are suppressed
• The replicas are also still used in the KafkaClusterCreator -> this piece was missed in Remove ZooKeeper support - Part I #10964. As this will likely have a larger refactoring, for the time being I just suppressed the deprecation warning and will address it in a follow-up PR.
• The generated CRD reference in the docs has been updated, but the docs themself will be updated only in one of the follow up PRs.

Checklist

• Write tests
• Make sure all tests pass
• Update documentation
• Try your changes from Pod inside your Kubernetes and OpenShift cluster, not just locally

[scholzj]

/azp run regression

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[scholzj]

/azp run upgrade

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[scholzj]

/azp run feature-gates-regression

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[scholzj]

(The OPA STs failure is not related to this PR but to #10980)

[ppatierno]

LGTM.