Add upgrade task for Solidus 3.0

The migration task to 3.0 includes the task for populating the
`spree_address.name` field. It is recommended that this task is
run before upgrading to Solidus 3.0, but when the app is updated
to the latest 2.11.x version.

core/lib/tasks/upgrade.rake

@@ -8,5 +8,12 @@ namespace :solidus do
       ] do
       puts "Your Solidus install is ready for Solidus 2.11.0"
     end
+
+    desc "Upgrade Solidus to version 3.0"
+    task three_point_zero: [
+        'solidus:migrations:migrate_address_names:up'
+      ] do
+      puts "Your Solidus install is ready for Solidus 3.0"
+    end
   end
 end