[BUGFIX] Prevent PHP Warning in CKEditor5Migrator

In CKEditor 4 the option "removeButtons"
can also be a comma separated list of strings.
CKEditor5Migrator now respects the old
string value and converts it to an array
before migrating the value.

Resolves: #100826
Releases: main, 12.4
Change-Id: Id6da8ea7daad70c2dc1690b44e26e383e9d7d180
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78960


Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>