Use config.govuk_time_zone

govuk_app_config insists on this now.

We could remove this config, as London is already the default, but
Guido says explicit is better than implicit.

config/application.rb

@@ -13,7 +13,7 @@
 module SearchApiV2
   class Application < Rails::Application
     config.load_defaults 7.0
-    config.time_zone = "London"
+    config.govuk_time_zone = "London"
     config.api_only = true
 
     # Google Discovery Engine configuration