[TASK] Change page unavailable status code from 500 to 503

Page unavailable (e.g. maintenance mode) is a "503 service unavailable"
error, not 500 internal server error.
If TYPO3 is in maintenance mode then a 503 error is now thrown.
Also 503 Error Handlers are taken into account now.

Releases: master, 10.4
Resolves: #93032
Change-Id: I8bd013fc202b07263388fc22ec2256262c40b709
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67074


Tested-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>