[TASK] ext:beuser BackendUser model does not extend extbase model

To further clean up ext:beuser and allow more refactoring
of the extension, the BackendUser model no longer extends
the ext:extbase base BackendUser model.
The patch is a straight merge of the lower class for now
and further patches will follow.

Resolves: #94614
Releases: master
Change-Id: I79978cdae8538ce04b15bc703ad0eefc415f6d27
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70025


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>