[TASK] Centralize Backend User Session Repository

Logic around Backend User Sessions and its access
across the EXT:beuser backend module is now encapsulated
and streamlined:

* The BackendUserSessionRepository does not inherit
from Extbase anymore, as there is nothing needed here
* No ObjectManager is needed, GeneralUtility can be used instead
* All access to the actual sessionBackend is now placed inside
this method.

This allows to further streamline the User Session handling
at a later stage.

Resolves: #93017
Releases: master
Change-Id: I02ab22d90e570cd04f4e6a3ea96412dfbf247967
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67045


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>