Rails5 fix AddMilestoneToLists migration rollback deleting wrong foreign key

ee/changelogs/unreleased/rails5-fix-migration-add-milestone-to-lists.yml

+---
+title: Rails5 fix AddMilestoneToLists migration rollback deleting wrong foreign key
+merge_request: 6865
+author: Jasper Maes
+type: fixed

ee/db/migrate/20180724161450_add_milestone_to_lists.rb

 class AddMilestoneToLists < ActiveRecord::Migration
   DOWNTIME = false
 
-  def change
+  def up
     add_reference :lists, :milestone, index: true, foreign_key: { on_delete: :cascade }
   end
+
+  def down
+    remove_reference :lists, :milestone, foreign_key: true
+  end
 end