[TASK] Remove Build/phpstan.level2.neon

Since all level 2 rules are active and only level 2
parameters are set to different values compared to
their originally declared value, the whole file can
be removed, the overall phpstan level can be raised
to 2 and the parameters can be defined in the main
phpstan file.

Releases: master, 10.4
Resolves: #92424
Change-Id: I8404f5f2f4ff318bb3419ed307b1ffe7edcc6822
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65902


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Alexander Schnitzler <git@alexanderschnitzler.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Alexander Schnitzler <git@alexanderschnitzler.de>