[TASK] Use TSFE->isBackendUserLoggedIn() instead of public variable

The public method TSFE->isBackendUserLoggedIn() instead of
TSFE->beUserLogin (public property).

Resolves: #83785
Releases: master
Change-Id: I1e54b7365b3b6810b606e93b0f888a7790492014
Reviewed-on: https://review.typo3.org/55575


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Tobi Kretschmann <tobi@tobishome.de>
Tested-by: Tobi Kretschmann <tobi@tobishome.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>