[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/+/74945
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/core/Classes/Utility/GeneralUtility.php

@@ -1696,7 +1696,7 @@ public static function minifyJavaScript($script, &$error = '')
      * If you are having trouble with proxies when reading URLs you can configure your way out of that with settings within $GLOBALS['TYPO3_CONF_VARS']['HTTP'].
      *
      * @param string $url File/URL to read
-     * @return mixed The content from the resource given as input. FALSE if an error has occurred.
+     * @return string|false The content from the resource given as input. FALSE if an error has occurred.
      */
     public static function getUrl($url)
     {