iFrame stops working

Adding a patch for 2.0.x branch.

Hi everyone,

I've just tested this locally with the following versions:

• collapse_text:2.0.x-dev:ebceea31
• drupal/core:10.2.2
• CKEditor5

I created a node with Full HTML text format with the following HTML markup:

[collapse]
<h3>
    Title heading with <em>emphasis</em> and <a href="#">some link</a>
</h3>
    Example text with markup.
&nbsp;
<iframe style="border:2px solid red;" src="https://[LOCAL_DEV_SITE_URL]/" title="Iframe Example" width="600px" height="400px"></iframe>
[/collapse]

 
and the iframe tag seemed to display fine, see the resulting screenshot below with the page, CKEditor5 and view source displays:

Not sure if the patch is still needed, so let's keep this issue open a bit longer and see if we can get any more feedback.
Given the security implications, as mentioned above at #2, ideally, we would prefer keeping the <iframe> tag in the allowed tags filter settings, thus not allowed by default (subject to restrictions of the text format).

In any case, thanks a lot to everyone for your contributions, patches, help testing, reviews and comments, it's greatly appreciated.

Feel free to let us know if you're still having issues with <iframe> tags, or if you have more questions, concerns or suggestions on any of the recent code changes, this ticket or the module in general, we would surely be glad to help.
Thanks in advance!