Add ckeditor5-stylesheets: false to Claro and Olivero (and fix it 😅)

Problem/Motivation

Claro and Olivero are adding front-end styles inside the CKEditor 4 iframe. With CKEditor 5, those styles need to be written so that they are specifically catering CKEditor 5, since CKEditor 5 is not loaded in an iframe. Decide how much of this CSS should be rewritten for CKEditor 5.

Until this is fixed, we'll see this in the Text Formats & Editors UI:

Proposed resolution

Since CKEditor 5 already ships with modern design out-of-the box, the proposed solution is to not rewrite any of the CKEditor 4 CSS for CKEditor 5 in any of the themes (at least for now).

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet