[TASK] Mark various code related to Backend user / DataHandler internal

In order to build a stronger Permission Handling system and
a stronger API for TCA resolving in TYPO3 v11, several methods and public
properties have been marked as "internal". These continue to be working
as is for TYPO3 v10 LTS, but are now marked as internal - most of the code
has been coming from PHP4 where no visibility restrictions have been available
back then.

Resolves: #91095
Releases: master
Change-Id: Ibb47739a0cdec5eae19fafa2e376eeb609aa6c53
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64162


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>