Specifiy coder for serialize attributes.

This is mandatory for Rails 7.1.
https://edgeapi.rubyonrails.org/classes/ActiveRecord/AttributeMethods/Serialization/ClassMethods.html

core/app/models/spree/preference.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class Spree::Preference < Spree::Base
-  serialize :value
+  serialize :value, coder: YAML
 
   validates :key, presence: true, uniqueness: { allow_blank: true, case_sensitive: true }
 end

core/app/models/spree/return_item.rb

@@ -67,7 +67,7 @@ class ReturnItem < Spree::Base
     scope :exchange_processed, -> { where.not(exchange_inventory_unit: nil) }
     scope :exchange_required, -> { exchange_requested.where(exchange_inventory_unit: nil) }
 
-    serialize :acceptance_status_errors
+    serialize :acceptance_status_errors, coder: YAML
 
     delegate :eligible_for_return?, :requires_manual_intervention?, to: :validator
     delegate :variant, to: :inventory_unit

core/lib/spree/preferences/persistable.rb

@@ -7,7 +7,7 @@ module Persistable
 
       included do
         include Spree::Preferences::Preferable
-        serialize :preferences, Hash
+        serialize :preferences, Hash, coder: YAML
         after_initialize :initialize_preference_defaults
       end