Rails 6.1.3.2 breaks Administrate

Created by: swalberg

• What were you trying to do?

Upgrade to the latest version of Rails

• What did you end up with (logs, or, even better, example apps are great!)?

An ActionView::Template::Error (see below)

• What versions are you running?
  • Rails 6.1.3.2
  • administrate 0.15.0

https://github.com/rails/rails/commit/c4c21a9f8d7c9c8ca6570bdb82d64e2dc860e62c requires that things passed to route helpers be symbols and not strings. https://github.com/thoughtbot/administrate/blob/main/app/views/administrate/application/index.html.erb#L44 and L56 break with

     ActionView::Template::Error:
       Please use symbols for polymorphic route arguments.

I'm not sure how prevalent this is within Administrate since I was just testing the simple show case when I tripped on this.