[BUGFIX] Make GeneralUtility::getFilesInDir() correctly check extensions

Invoke `str_ends_with` on lowercased filesystem entries in order to check
for allowed extensions. This is more stable than `substr`ing the
comma-separated list of extensions.

Resolves: #101035
Releases: main, 12.4
Change-Id: I6a0eee1f6ddb7406a8da168ed489aba6bfc83263
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80230


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>