rails: update settings to 6.1 defaults.

Most of the new settings aren't relevant to us. We do have to fix some
tests to work around a Rails bug. `assert_enqueued_email_with` uses the
wrong queue, so we have to specify it explicitly. This is fixed in Rails
HEAD but not yet released.

config/application.rb

@@ -35,7 +35,7 @@ module Danbooru
     config.app_generators.scaffold_controller :responders_controller
 
     # Initialize configuration defaults for originally generated Rails version.
-    config.load_defaults 6.0
+    config.load_defaults 6.1
     config.active_record.schema_format = :sql
     config.encoding = "utf-8"
     config.filter_parameters += [:password, :password_confirmation, :password_hash, :api_key]