DefaultMailArchive component should not manipulate logback logger directly

To manage the "debug mode", this component manipulates log level of logback implementation directly, that is ugly.
Starting from XWiki 4.2.M2, the LoggerManager allows to do that. Even if only implementation for now is logback, it would be far better to rely on LoggerManager to achieve this change of log level.

Of course, the mail archive must depend on XWiki >= 4.2.M2 in order to achieve this task.