[TASK] Declare PHP 8 compatibility

composer req php:"^7.4 || ^8.0"

Note this declares core master PHP 8 compatibility,
even though core is not yet compatible in practice.
The situation should settle until 11.2, though.

Resolves: #93612
Releases: master
Change-Id: Ibd428260ebd48936bb193b1df747283d3b35f410
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68150


Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Richard Haeser <richard@richardhaeser.com>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Richard Haeser <richard@richardhaeser.com>