Better method of showing forum containers in form

Forum is approved for removal. See #1898812: [policy] Deprecate forum module for removal in Drupal 11 →

This is now Postponed. The status is set according to two policies. The Remove a core extension and move it to a contributed project → and the Extensions approved for removal → policies.

It will be moved to the contributed extension → once the Drupal 11 branch is open.

Adding the Accessibility tag as this issue has implications for screen reader users who can't see any text decoration that may indicate that a forum container is different to forums in the select field. As the screen reader announces the forum container as a selectable item, this leads to confusion and a poor user experience.

I appreciate that this issue is currently marked as postponed, but adding the tag for when the forum module is split off and issues start to be addressed again.

Now we're outside core I think it's fine to close this in favour of hierarchical select or shs