[TASK] Avoid last usage of GU::_GPmerged() and deprecate

After years of refactoring, TYPO3 core finally removes
a last usage of GeneralUtility::_GPmerged() which is
an anti-pattern method, since it accesses $_GET
and $_POST instead of retrieving things from
PSR-7 ServerRequestInterface $request.

Resolves: #99615
Releases: main
Change-Id: I3cb6f3a0cee0a7654dce331dc40ab51f2efe41f4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77477


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>