WCAG 3.3.1 (Level A) / 4.1.3 (Level AA): Field input error is not announced in full - Missing `aria-atomic` on assertive live region

[bspeare]

📝 Reproduction steps

1. Run Siteimprove plugin on https://ckeditor.com/docs/ckeditor5/latest/examples/builds/classic-editor.html.
2. Look for "Field input error is not announced in full" A and AA in the Issues list.
3. Highlight the code in the developer tools.

✔️ Expected result

The assertive live region rendered should include aria-atomic="true":

❌ Actual result

The live region is rendered without aria-atomic

❓ Possible solution

Include aria-atomic="true" in https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-ui/src/arialiveannouncer.ts.

📃 Other details

Flagged element in DOM:

<div aria-live="assertive" aria-relevant="additions">
  <ul class="ck ck-aria-live-region-list"></ul>
</div>

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