Important: #105175 - Move FrontendBackendUserAuthentication into EXT:frontend

See forge#105175

Description

The internal \TYPO3\CMS\Frontend\Authentication\FrontendBackendUserAuthentication class, used for frontend requests while being logged in the backend has been moved from EXT:backend to EXT:frontend, since its dependencies are limited to EXT:core and EXT:frontend.

While for v13 a class alias mapping and a legacy notation for IDE's is available, the class is marked as @internal and therefore does not fall under TYPO3's Core API deprecation policy.