[BUGFIX] update categories in flexforms

Since the categories are now managed with sys_category,
the existing flexforms need to be updated as well.

Resolves: #58807
Change-Id: Id4206cc490cecd44ae98563deee2592ece3b4edf
Reviewed-on: https://review.typo3.org/30138
Reviewed-by: Frans Saris
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer