[TASK] Mark various obvious code as @internal

All specific controllers for specific Backend actions, Backend-module related modules,
all hook implementations (where the core uses hooks by itself), and module-specific
ViewHelpers are now marked as @internal to ensure developers what is
part of the public TYPO3 Core API.

within
- EXT:backend (except ViewHelpers)
- EXT:beuser
- EXT:about
- EXT:belog

All @api methods have been removed.

Resolves: #86505
Releases: master
Change-Id: I1fd1e16f7885e81fb994999e5873a1a3a1aa6c3b
Reviewed-on: https://review.typo3.org/58524


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>