[TASK] Raise PHP requirement to 7.2.0

As decided by core team leader, PHP minimum version
requirement for core version 9.0 is 7.2.0.

The patch sets this in root composer.json, together
with config.platform.php, and adapts sysext/core/composer.json.

Resolves: #83290
Related: #83289
Releases: master
Change-Id: I813d0a2b69005c84ba5dedb45b3d4bd0388e43d8
Reviewed-on: https://review.typo3.org/55035


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>