[TASK] Deprecate PageRepository->getExtURL()

PageRepository->getExtURL() is a pretty upgly
public helper method that has a depedency to
$GLOBALS['TYPO3_REQUEST']. It is used only once.
The patch moves the method body to the consuming
class and deprecates it.

Resolves: #99558
Releases: main
Change-Id: I7c1e8d6b1a454950de6fa7469d13fbab81ee2962
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77330


Reviewed-by: Jasmina Ließmann <minapokhalo+typo3@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Jasmina Ließmann <minapokhalo+typo3@gmail.com>
Tested-by: core-ci <typo3@b13.com>