[TASK] Harden array callbacks in EXT:backend

To improve code readability and to help static analysis, add
type annotations and use first-class callables.

Resolves: #100330
Releases: main
Change-Id: I09a354bb99823a621e6ada5234243e44e2fd680a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78283


Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>