[BUGFIX] Use CKEditor5 GHS attribute for CSS classes in link plugin

The GHS (General HTML Support [1]) attribute `htmlA` is used by the
style plugin to apply or remove style classes. Make sure to use the
same attribute interface in order for CSS class usage to be interopable
across our set of CKEditor5 plugins.

Note that our custom `linkClass` attribute prevented the CKEditor5
style plugin from parsing existing classes, as this plugin expects
CSS classes to be available on the `htmlA` attribute (defined in
the nested `classes` array).

[1] https://ckeditor.com/docs/ckeditor5/latest/features/html/general-html-support.html

Resolves: #101429
Resolves: #101427
Releases: main, 12.4
Change-Id: I704982cd950f0c7cacb1c99bac6a70f417caf40a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81051


Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Andre...