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This is a port of #3199 to the 2.24.x branch.

Removes weak references to `Logger`s in `LoggerRepository`.
The usage of weak references in `LoggerRepository` might cause `null` to be returned by `LogManager.getLogger()` of all Log4j Core versions up to `2.24.1`.
Versions of Log4j API up to `2.24.0` did hold **hard** references to all the registered loggers, so the change will not alter the previous behavior.

This PR also inverts the order of the `String` and `MessageFactory` keys to the `LoggerRepository` multi-map to limit the number of internal maps. The external map is a `WeakHashMap` to allow logger-specific message factories to be GC-ed.

Closes #3143.

Co-authored-by: Volkan Yazıcı <volkan@yazi.ci>

This backports #3050 to the 2.24.x branch.

Co-authored-by: Volkan Yazıcı <volkan@yazi.ci>

Fixes #3194, ports #3212.