Support importmap-rails 2.0

Refs. #3666

Author: mshibuya

app/views/layouts/rails_admin/_head.html.erb

@@ -19,8 +19,8 @@
   <%= stylesheet_link_tag "rails_admin.css", media: :all, data: {'turbo-track': 'reload'} %>
   <%= javascript_inline_importmap_tag(RailsAdmin::Engine.importmap.to_json(resolver: self)) %>
   <%= javascript_importmap_module_preload_tags(RailsAdmin::Engine.importmap) %>
-  <%= javascript_importmap_shim_nonce_configuration_tag %>
-  <%= javascript_importmap_shim_tag %>
+  <%= javascript_importmap_shim_nonce_configuration_tag if respond_to? :javascript_importmap_shim_nonce_configuration_tag %>
+  <%= javascript_importmap_shim_tag if respond_to? :javascript_importmap_shim_tag %>
   <%= # Preload jQuery and make it global, unless jQuery UI fails to initialize
       tag.script "import jQuery from 'jquery'; window.jQuery = jQuery;".html_safe, type: "module" %>
   <%= javascript_import_module_tag 'rails_admin' %>