Previously, when LogbackLoggingSystem or Log4JLoggingSystem were
initialized during application start up, they would install SLF4J’s Java
logging bridge handler, however no corresponding uninstall was performed
during application shutdown. When deployed to a servlet container, where
the application’s lifecycle doesn’t match the JVM’s lifecycle, this lead
to a memory leak.

This commit updates LoggingSystem to introduce a new cleanUp method. An
empty implementation is provided to preserve backwards compatibility
with existing LoggingSystem subclasses. Both LogbackLoggingSystem and
Log4JLoggingSystem have been updated to implement cleanUp and uninstall
the SLF4J bridge handler. LoggingApplicationListener has been updated
to call LoggingSystem.cleanUp in response to a ContextClosedEvent.

Closes gh-2324