[TASK] Declare all controllers of ext:backend as strict typed

Set all missing files to declare(strict_types=1);

Resolves: #96790
Releases: main
Change-Id: I347263bf182200c304c5f4d841e93738f759ba0f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73358


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>