[FEATURE] PHP 8.0 support

After a long way of fixing our code base to have
all our tests (unit, functional, acceptance tests)
green, TYPO3 now officially runs with PHP 8.0.

TYPO3 v11 LTS thus will support PHP 7.4 and PHP 8.0
(and hopefully PHP 8.1 once the dependencies support PHP 8.1).

There might be a few PHP notices throughout TYPO3 Frontend
and Backend, but all base functionality is now available
for PHP 8.0 as well.

Resolves: #93631
Related: #94057
Releases: master
Change-Id: I5798e6f49ee637b794df096fd44b8157d96b74bc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69489


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>