[BUGFIX] Remove deprecated reflection API usage

PHP 8 deprecated a number of very old bits of the Reflection API. This
patch updates ClassSchema to use the newer equivalents.

Also fixes some unrelated PHP 8.0 errors around undefined value handling.

Resolves: #94001
Releases: master, 10.4
Change-Id: Id235fa830894e9a2dc6465444ec2c6808f18c3aa
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68898


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>