[TASK] Make use of PSR-7 request object in BE Template module

Switch from GeneralUtility::_GP() to PSR-7 in EXT:tstemplate.

All occurrences are replaced by proper Object-Oriented coding.

Resolves: #88582
Releases: master
Change-Id: I4cb94601f1c6bbe62025f40f92154f40ec051ce8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60993


Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>