[TASK] Improve type annotations for GU::locationHeaderUrl

This helps PHPStan better analyse code that uses this method.

Resolves: #99133
Releases: main, 11.5
Change-Id: Ia6da1e1f25a9bfd0f493b2c1e244d79aab865c56
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76698


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>