[DOCS] Update CKEditor5 CSS styling limitations section

With #100768 the CKEeditor5 CSS scoping was migrated from server-side
SCSS preprocessing to a JavaScript based CSS prefixer. The Feature.rst
has not been updated and still referred to limitations caused by the
removed SCSS parser.

Resolves: #103151
Related: #100768
Releases: main, 12.4
Change-Id: I0f9e7d0c8c88c906b65290c29f2b2cc0e179058a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82985


Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>