[TASK] Improve return type for GeneralUtility:getUrl

Set the return type to string|false instead of mixed.

Resolves: #97832
Releases: main, 11.5
Change-Id: I8648392bb8b8d95aaf8ab5416d9e7d1043f8e9b0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75017


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