[TASK] Deprecate GeneralUtility::_GP()

After just a couple of years streamlining the core
request handling, we finally have no non-deprecated
usages to GeneralUtility::_GP() anymore and can
happily deprecate it.

Resolves: #100053
Related: #99615
Related: #99633
Releases: main
Change-Id: Ib1acb97b5fd8e66aa3bcc285b01a93f31964c0cb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77990


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>