[TASK] Replace prophecy in EXT:core BackendUserAuthenticationTest.php

Resolves: #98757
Releases: main
Change-Id: Ieea7e9fd217c6908adc7c1837e396ddc5fd3d5f6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76346


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Torben Hansen <derhansen@gmail.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Torben Hansen <derhansen@gmail.com>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>