[BUGFIX] Fixes required CKeditor fields in flexforms

CKeditor throwed errors in javascript, when field identifier
contained dot's. Using array syntax for CKEDITOR.instances
solved this issue.

Releases: master, 8.7
Resolves: #81390
Change-Id: I1f876de7aab1b6370aa89420600079c1439d339d
Reviewed-on: https://review.typo3.org/53490


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>