Rename unprocessable_entity to unprocessable_content

When the 422 status was initially proposed in
[RFC4918](https://datatracker.ietf.org/doc/html/rfc4918) (2007), it was
called 'Unprocessable entity'.

When it became a standard in
[RFC9110](https://datatracker.ietf.org/doc/html/rfc9110) (2022), it was
renamed 'Unprocessable content'.

Rack now supports it as `:unprocessable_content` and has deprecated
`:unprocessable_entity`. This commit updates it to use the new name.

Author: DavidBiddle

app/controllers/application_controller.rb

@@ -65,6 +65,6 @@ def missing_parameter(exception)
   end
 
   def invalid_record(resource)
-    render json: resource.record.errors, status: :unprocessable_entity
+    render json: resource.record.errors, status: :unprocessable_content
   end
 end