[TASK] Enable phpstan rules up to level 8

As there are a lot of rules that do not raise any errors
with our code base, those rules up to level 8 are enabled
with this patch resulting in more pre merge tests being
executed as of now.

Releases: master
Resolves: #90795
Change-Id: I3628569c90e1e7bbf2ffde81abbef37065bd45e8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63812


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
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>