Use Spree::Event.activate_all_subscribers when available

The new method `Spree::Event.activate_all_subscribers` is introduced
in Solidus with solidusio/solidus#3758 as a better way to subscribe
event subscribers. Under the hood, event subscribers use new interal
mappings.

Also, we need to keep around the old interface as well until it won't
be supported anymore in Solidus.

lib/solidus_support/engine_extensions.rb

@@ -36,7 +36,12 @@ def load_solidus_subscribers_from(path)
           path.glob("**/*_subscriber.rb") do |subscriber_path|
             require_dependency(subscriber_path)
           end
-          Spree::Event.subscribers.each(&:subscribe!)
+
+          if Spree::Event.respond_to?(:activate_all_subscribers)
+            Spree::Event.activate_all_subscribers
+          else
+            Spree::Event.subscribers.each(&:subscribe!)
+          end
         end
       end