[TASK] Simplify setting BE/warning_mode

The setting used bits 1 and 2. But useful values were only 0/1/2.
So this setting can be simplified.

Resolves: #85508
Releases: master
Change-Id: Ib78174035bcefdec7658f37dae9071062dcb52c1
Reviewed-on: https://review.typo3.org/57500


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>