Use Drupal's new iframe render component

Problem/Motivation

Added in Drupal 10.1, the new iframe render component → provides a slightly different syntax and the ability to include lazy loading. It would be good to make this switch to see if we enjoy it better.

Proposed resolution

Rewrite the use of the html_tag component found in PreviewPaneController, line 50, to use the new iframe component.

Remaining tasks

Do it, write tests.