We change the default value of kafka.legacy_kafka_client_enabled from
True to False. The use of new Confluent Kafka client is recommended.
DevStack plugin does not set this option anymore.
Depends-On: https://review.opendev.org/740959
Depends-On: https://review.opendev.org/740966
Change-Id: I4d57b8893a6a131769009dc3299789d3fc89bab6
Story: 2007924
Task: 40338