Use Spree::Event.subscribe_subscribers! when available

The new method `Spree::Event.subscribe_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?(:subscribe_subscribers!)
+            Spree::Event.subscribe_subscribers!
+          else
+            Spree::Event.subscribers.each(&:subscribe!)
+          end
         end
       end