DEV: correct Rails 6 deprecation

DEV: correct Rails 6 deprecation

render template is backwards compatible with Rails 5, code is simpler.

Thanks @jhawthorn, this is our last Rails 6 deprecation

diff --git a/app/controllers/metadata_controller.rb b/app/controllers/metadata_controller.rb
index 299fc2d..8148b9c 100644
--- a/app/controllers/metadata_controller.rb
+++ b/app/controllers/metadata_controller.rb
@@ -9,7 +9,7 @@ class MetadataController < ApplicationController
   end
 
   def opensearch
-    render file: "#{Rails.root}/app/views/metadata/opensearch.xml"
+    render template: "metadata/opensearch.xml"
   end
 
   private

GitHub sha: 16ef060a