[TASK] Deprecate FrontendBackendUserAuthentication methods

The last methods within FrontendBackendUser (BE_USER in FE mode)
are now deprecated in favor of Admin Panel and middlewares:
- checkBackendAccessSettingsFromInitPhp()
- extPageReadAccess()
- extGetTreeList()
- extGetLL()

Resolves: #86288
Releases: master
Change-Id: I6752589a4b7a5353ae1699f3786be743c6f1f13d
Reviewed-on: https://review.typo3.org/58305


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>