[upstream] [GHS] CKEditor 5 removes <a>s that wrap HTML elements not natively supported by CKEditor 5

Problem/Motivation

Reported by @glynster at #3337298-12: [upstream] [GHS] CKEditor 5 removes empty inline elements:

CKeditor source view:

<div class="adblock">
  <a href="/link">
    <picture>
      <source media>
    </picture>
  </a>
</div>

CKeditor default view:

<div class="adblock">
  <picture>
    <source media="">
  </picture>
</div>

And indeed dataloss. We migrated to CKeditor 5 and just notcied all links were stripped from our adblocks.

Steps to reproduce

Copy/paste that first snippet into https://ckeditor.com/docs/ckeditor5/latest/features/html/general-html-su...'s source view
turn off source view
return to source view

The wrapping <a> is indeed lost!

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet