DEV: Migrate about config area to Form Kit (#28021)

Form Kit is our new form library/framework for unifying the way forms look across Discourse. The admin config area for the /about page is a new form that isn't currently used, so it makes sense for it to be one of the first forms to be migrated to Form Kit to test the library.

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

spec/system/page_objects/components/form_kit.rb

@@ -26,6 +26,8 @@ module PageObjects
           component.find(".fk-d-menu__trigger")["data-value"]
         when "select"
           component.find("select").value
+        when "composer"
+          component.find("textarea").value
         end
       end