[upstream] CKEditor 5 moves <span> inside <a> tag

Problem/Motivation

Ckeditor moves span around by itself.

Steps to reproduce

Pasting this inside the source code view and switching back to wysiwyg mode.
<span class="ctr-box"><a class="ctr-button" href="https://www.drupal.org" rel="noopener nofollow" target="_blank">FOO</a></span>

Will result in :

<p>
    <a class="ctr-button" href="https://www.drupal.org" target="_blank" rel="noopener nofollow"><span class="ctr-box">FOO</span></a>
</p>

The span tag is moved inside the a tag. Kills the styling and logic of many of my templates.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet