[FIX JENKINS-40266] Allow override UserProperty.setUser(User) (#2655)

UserProperty may contain nested objects that depend on User. 
On User reconfiguration setUser(User) is called so it should be non-final to have ability override it and update references in nested objects.

Signed-off-by: Kanstantsin Shautsou <kanstantsin.sha@gmail.com>