GH-1141: Streams cleanup docs
GH-1141: Streams cleanup docs

Clarify streams cleanup default in the docs.
Effective from Spring Kafka 2.7, no cleanup will be performed on shutdown.

Resolves https://github.com/spring-cloud/spring-cloud-stream-binder-kafka/issues/1141

spring-cloud/spring-cloud-stream,garyrussell/spring-cloud-stream,garyrussell/spring-cloud-stream,spring-cloud/spring-cloud-stream,spring-cloud/spring-cloud-stream,garyrussell/spring-cloud-stream