Use JWK thumbprint as the kid

The kid is used to identify individual keys returned by the JWKS
endpoint. As per the documentation for the gem
(https://github.com/jwt/ruby-jwt) we can use the standardised JWK
thumbprint as the kid for the JWK rather than the legacy custom
approach the gem uses by default.

Author: stephencdaly

config/application.rb

@@ -89,5 +89,7 @@ class Application < Rails::Application
 
     I18n.available_locales = %i[en cy]
     I18n.default_locale = :en
+
+    JWT.configuration.jwk.kid_generator_type = :rfc7638_thumbprint
   end
 end