Decrease logging level to the WARN

https://build.spring.io/browse/AMQP-SA17X-120

There is some race condition in the Log4J that log messages
are processed on a wrong thread.

*  Decrease logging level to the `WARN` for the `spring-rabbit` to
minimize the level of vulnerability for the `AmqpAppenderIntegrationTests`

spring-rabbit/src/test/resources/log4j-amqp.properties

@@ -31,6 +31,6 @@ log4j.appender.amqpXml.contentType=text/xml
 log4j.category.org.springframework.amqp.rabbit.log4j=DEBUG, amqp
 log4j.category.org.springframework.amqp.rabbit.logging.customLayout=DEBUG, amqpXml
 
-log4j.category.org.springframework.amqp.rabbit=DEBUG
+log4j.category.org.springframework.amqp.rabbit=WARN
 log4j.category.org.springframework.beans.factory=INFO