Update `Log4J2LoggingSystem` so that call to `setLevel` with a `null`
level with remove the logger if it was previously configured by a
`LoggingSystem` call.

To track which loggers have been configured by us, and which have been
configure directly by the user, a custom `LoggerConfig` subclass is
used. We'll only remove `LevelSetLoggerConfig` classes, for any others
we'll call `setLevel(null)` on the config.

Prior to this commit, it was impossible to set then reset a logger
level using the actuator endpoint. This is because Log4J doesn't provide
a way to get the actual configured level. If the `setLevel(null)` has
been applied, then `getLevel()` will return the value of the parent
logger or a default value of `ERROR`.

Fixes gh-24298