replace innerHTML with setHTMLUnsafe in ajax.js

https://git.drupalcode.org/project/drupal/-/merge_requests/12324.diff

not quite sure about the failing test. Doesn't seem related to my change?
Do you have a doc link on how to run these locally with more debug info?

An Ajax test failed, which seems related. Automated test documentation is on drupal.org.

If this problem was discovered on a version of Drupal that is not 11.x, add that information in the issue summary and leave the version at 11.x. In Drupal core changes are made on on 11.x (our main development branch) first, and are then back ported as needed according to the Core change policies → . Also mentioned on the version → section of the list of issue fields documentation.

alright thank you @quietone. I'll try to figure out the failing tests.