Rails 7.1: set up Active Record serialization

Having added encryption changes, other specs (HistoricalEvent being one
began failing due to initialization order. Prior to 7.1, YAML was the
default serializer for calls like `serialize :new_value`.

Set a global YAML default to preserve behaviour.

config/application.rb

@@ -60,6 +60,9 @@ class Application < Rails::Application
 
     config.active_job.queue_adapter = :sidekiq
 
+    # Set up YAML as the default serializer from Rails 7.1
+    config.active_record.default_column_serializer = YAML
+
     config.active_record.yaml_column_permitted_classes = [
       ActiveSupport::TimeWithZone,
       ActiveSupport::TimeZone,