Verify monitor appender shuts down thread pool on config change

In #4879 changes were made to the monitor appender to use a thread pool. These changes added code that was supposed to shut the thread pool down when log4j config changed and new appender was created. As a follow on task to #4879 need to manually verify this code is working. One attempt was made as explained in the following comment.

_Originally posted by @keith-turner in https://github.com/apache/accumulo/pull/4879#discussion_r1769356294_

Review the monitor appender changes from #4879 and the referenced pull-request discussion first. Exercise a Log4j configuration change that creates a new appender, and verify that the previous appender's thread pool shuts down; done means the shutdown behavior is manually confirmed.

Written by the indexing model from the issue text.