Deprecate usage of TCCL in `Loader/LoaderUtil`

As a complement to #1977, we should probably remove the usage of the thread context classloader from `Loader` and `LoaderUtil` in `2.x`

The usage of [`Loader.loadClass(String)`](https://logging.apache.org/log4j/2.x/javadoc/log4j-core/org/apache/logging/log4j/core/util/Loader.html#loadClass(java.lang.String)) and [`LoaderUtil.loadClass(String)`](https://logging.apache.org/log4j/2.x/javadoc/log4j-api/org/apache/logging/log4j/util/LoaderUtil.html#loadClass(java.lang.String)) and similar methods is extremely prone to memory leaks, since the result of such a call is often assigned to a static field.

The thread context classloader is often the classloader of a web application that can be stopped and restarted at will. Any object that we obtain from it must be kept in a weak reference.

IMHO, we should always provide an explicit classloader to load a class by name and we can only choose between:

- the classloader that loaded `log4j-core` or `log4j-api`,
- the classloader associated with the current `LoggerContext`.