[BUGFIX] Ensure AbstractExceptionHandler::IGNORED_EXCEPTION_CODES is protected

With #100720 the AbstractExceptionHandlers
const IGNORED_EXCEPTION_CODES was set from private
to public in order to allow to use it in subclasses.

However, public is making this a full public API,
which should be within the ExceptionHandler logic
in general.

For this reason, the constant is now changed from
public to protected.

Resolves: #100736
Relates: #100720
Releases: main, 11.5
Change-Id: I58427977c2217792ae854738f59527bdb2e8339e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78736


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>