[BUGFIX] Fix return type hint for retrieveYamlFilesFromExtensionFolders

The method retrieveYamlFilesFromExtensionFolders actually returns an array
of strings and not an array of File objects.

Releases: master, 10.4
Resolves: #92047
Change-Id: Ic031c28891612420c57c2caefc3608772806d7c7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65365


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>