[TASK] Streamline userid/username handling and system-maintainer checks

The new methods AbstractUserAuthentication::getUserName() and
AbstractUserAuthentication::getUserId() can be used to resolve
the corresponding values (instead of using the $user->user array).

In addition, the pure system-maintainer checks have been moved into
to central BackendUserAuthentication::isSystemMaintainer(). The term
"pure" refers to ignoring the development context and not applying
any fallbacks in case the setting is empty.

Resolves: #103323
Releases: main, 12.4
Change-Id: Ia7db222dac32acc2ef13a34ded4545ba1aedefc3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83306


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>