[BUGFIX] Force type in ckeditor configuration migration

To avoid exceptions during the ckeditor migration the type is forced
to be a string.

Resolves: #102521
Releases: main, 12.4
Change-Id: Icfdedb5b6ef74720e8f7da205a66115bc6d7a39a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81909


Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>