Print a deprecation warning if order is updated without validations

Also, this commit adds the new default value to the spree.rb
initializer, that will be used when installing new Solidus applications
and to the dummy app that will run specs with the new value set to the
new behavior's value.

core/lib/generators/solidus/install/templates/config/initializers/spree.rb.tt

@@ -34,6 +34,12 @@ Spree.config do |config|
   # See https://github.com/solidusio/solidus/pull/3456 for more info.
   config.raise_with_invalid_currency = false
 
+  # Set this configuration to `false` to avoid running validations when
+  # updating an order. Be careful since you can end up having inconsistent
+  # data in your database turning it on.
+  # See https://github.com/solidusio/solidus/pull/3645 for more info.
+  config.run_order_validations_on_order_updater = true
+
   # Permission Sets:
 
   # Uncomment and customize the following line to add custom permission sets

core/lib/spree/core/engine.rb

@@ -64,6 +64,15 @@ class Engine < ::Rails::Engine
             caller
           )
         end
+
+        if Spree::Config.run_order_validations_on_order_updater == false
+          Spree::Deprecation.warn(
+            'Spree::Config.run_order_validations_on_order_updater set to false is ' \
+            'deprecated and will not be possibile in Solidus 3.0. Please switch this ' \
+            'value to true and check that everything works as expected.',
+            caller
+          )
+        end
       end
 
       # Load in mailer previews for apps to use in development.

core/lib/spree/testing_support/dummy_app.rb

@@ -117,6 +117,7 @@ class Application < ::Rails::Application
 Spree.config do |config|
   config.mails_from = "[email protected]"
   config.raise_with_invalid_currency = false
+  config.run_order_validations_on_order_updater = true
   config.use_combined_first_and_last_name_in_address = true
 
   if ENV['ENABLE_ACTIVE_STORAGE']