[!!!][TASK] Add strict types to GU::fixed_lgd_cs

Internal calls to this method have already been adjusted by
previous patches.

Another one has been found in acceptance tests, which is now
fixed as well.

Resolves: #98511
Related: #95913
Related: #97887
Releases: main
Change-Id: Icf68ed4a05d761e0dcfff33dcb7670c80a934b37
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75950


Tested-by: core-ci <typo3@b13.com>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Kevin Appelt <kevin.appelt@icloud.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>