Monkey patch rails_erd to work with Rails 6.1

Until voormedia/rails-erd#358 is resolved,
rails-erd cannot generate ERD data from the models on Rails 6.1.

This patch reoslves the above by monkey-patching in a fix that has
been proposed upstream.

config/initializers/rails_erd.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+if Rails.env.development?
+  require 'rails_erd/domain'
+
+  module RailsERD
+    class Domain
+      def name
+        return unless defined?(Rails) && Rails.application
+
+        if Rails.application.class.respond_to?(:module_parent)
+          Rails.application.class.module_parent.name
+        else
+          Rails.application.class.parent.name
+        end
+      end
+    end
+  end
+end