DEV: Add admin warning for sites on Ember 3 (#25459)

Running Discourse 3.2 stable under Ember 3 will technically be possible, but is only intended as a short-term migration point. This commit adds an admin warning for sites which are using this configuration, to make it clear that themes and plugins are unlikely to support the configuration.

https://meta.discourse.org/t/287211

app/models/admin_dashboard_data.rb

@@ -219,7 +219,8 @@ class AdminDashboardData
                       :unreachable_themes,
                       :watched_words_check,
                       :google_analytics_version_check,
-                      :translation_overrides_check
+                      :translation_overrides_check,
+                      :ember_version_check
 
     register_default_scheduled_problem_checks
 
@@ -445,6 +446,10 @@ class AdminDashboardData
     nil
   end
 
+  def ember_version_check
+    I18n.t("dashboard.ember_version_warning") if ENV["EMBER_VERSION"] == "3"
+  end
+
   def out_of_date_themes
     old_themes = RemoteTheme.out_of_date_themes
     return unless old_themes.present?