[TASK] Add better PHPStan annotation for method getRepositoryClassName

Resolves: #103170
Releases: main, 12.4, 11.5
Change-Id: Ic7f63f6c55b832af27e9e30a0711ba042df42aa3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83056


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