[TASK] Use ckeditor's native readOnly mode

Textarea can be configured in TCA to be read only and to have a richtext
editor enabled. If `readOnly` is enabled, ckeditor is now rendered in a
read-only mode instead of rendering a plain textarea containing HTML
markup.

Resolves: #97296
Releases: main, 11.5
Change-Id: I9e457123811ef53ad4e3e355a68e3f1e5701dcfc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74143


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>