[TASK] Increase maxitems for settings.categories

Increase the maxitems value for settings.categories in
the flexforms from 20 to 99.

Change-Id: Icd17b741c32706d705b6be1b48b291216ec78fb0
Resolves: #51978
Reviewed-on: https://review.typo3.org/23844
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer