allow parent_name to be used in older ActiveSupport versions

Author: jcody

lib/promptify/railtie.rb

@@ -22,7 +22,12 @@ def show_pretty_prompt
     end
 
     def app_name
-      Rails.application.class.parent_module_name.underscore.dasherize
+      # ActiveSupport's `Module#parent_name` is deprecated in 6.1+.
+      if Rails.application.respond_to?(:parent_module_name)
+        Rails.application.class.parent_module_name.underscore.dasherize
+      else
+        Rails.application.class.parent_name.underscore.dasherize
+      end
     end
 
     def heroku_app