Enable deprecation warnings in test and development

Now that we've got rid of all the warnings we had, we can enable them so
we'll notice new warnings when we introduce them.

This was the default option until Ruby 2.7.2 was released [1]. These
warnings were turned off by default because pretty much every Ruby gem
had dozens of warnings with Ruby 2.7 due to the changes in the way Ruby
handles keyword arguments.

[1] https://www.ruby-lang.org/en/news/2020/10/02/ruby-2-7-2-released/

config/environments/development.rb

@@ -1,3 +1,5 @@
+Warning[:deprecated] = true
+
 Rails.application.configure do
   # Settings specified here will take precedence over those in config/application.rb.