[JENKINS-47718] - Deprecate User#getUser(String) (#3114)

* [JENKINS-47718] - Deprecate User#getUser(String)

`User#getUser(String)` method causes lots of confusion for plugin developers. It implicitly creates a user if it cannot be found, and in many cases this is actually a not-expected behavior. This change deprecates the method and to creates a new `User#getOrCreate()` which is more explicit

* [JENKINS-47718] - Address comments from @jglick