Type parameters used only in the return type

[ppkarwasz]

As described in TypeParameterUnusedInFormals, these methods provide developers a false sense of type safety, whereas in reality they can cause ClassCastExceptions at runtime:

• [ ] [Logger#getMessageFactory](https://logging.apache.org/log4j/2.x/javadoc/log4j-api/org/apache/logging/log4j/Logger.html#getMessageFactory()),
• [ ] ReadOnlyStringMap#getValue,
• [ ] LoaderUtil#newInstanceOf(String) and similar,
• [ ] ObjectThreadContextMap#getValue.

Changing/removing these methods is a binary compatible, but source incompatible change, hence this must be done in a major release.

[ppkarwasz]

This is related to #1850.

[jvz]

Also a few new places in the DI system (some of which is difficult to express within the limitations of the Java type system).

[jvz]

In fact, there are numerous places in the DI system abusing this. I'd be open to cleaning that up as it might also allow for the Key<T> class to be turned into a Key record class instead as only the Type reference is needed.