Default log levels are inconsistent between log4J2 and logback

[`log4J2.xml`](https://github.com/spring-projects/spring-boot/blob/ff3b05a421213b360473fd1a379d80d20c527611/spring-boot-project/spring-boot/src/main/resources/org/springframework/boot/logging/log4j2/log4j2.xml#L23) configures a default log level of `WARN` for `org.springframework.boot.actuate.endpoint.jmx` but `defaults.xml` for logback does not.