Major Description
There's an off-by-one error in the config check / id generation for max.reserved.broker.id setting. The auto-generation will generate max.reserved.broker.id as the initial broker id as it's currently written.
Not sure what the consequences of this are if there's already a broker with that id as I didn't test that behavior.
This can return 0 + max.reserved.broker.id:
https://github.com/apache/kafka/blob/8dbd688b1617968329087317fa6bde8b8df0392e/core/src/main/scala/kafka/utils/ZkUtils.scala#L213-L215
However, this does a <= check, which is inclusive of max.reserved.broker.id:
https://github.com/apache/kafka/blob/8dbd688b1617968329087317fa6bde8b8df0392e/core/src/main/scala/kafka/server/KafkaConfig.scala#L984-L986