See https://github.com/discourse/discourse-encrypt/pull/282
> `cooked` was an Ember SafeString. The internal storage of the string changed from `.string` to `.__string` at some point between Ember 3.28 and Ember 5. Instead, we can use `toString()` which is guaranteed to work in all situations
Most of the core plugins were already hidden, this hides
chat, styleguide, and checklist to avoid potential confusion
for end users.
Also removes respond_to? :hide_plugin, since that API has been
in place for a while now.
