[BUGFIX] Use 403 header instead of 401 header

The usage of a 401 header must be accompanied by a valid
www-authenticate header, which does not support form-based logins.

Resolves: #85411
Releases: master, 8.7
Change-Id: I71062c58a7d846214f1fec41e78cce4ae72955f3
Reviewed-on: https://review.typo3.org/57399


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Markus Klein <markus.klein@typo3.org>