Stop generating code to autoload overrides

Up to this PR, when installing Solidus, we're adding code to the host
application to load files matching `app/**/*_decorator*.rb`.

There exist a series of issues about that:

- It goes against current recommendations on the Rails guides, where
overrides are placed under `app/overrides/**/_*override.rb` ( see
https://guides.rubyonrails.org/engines.html#improving-engine-functionality).

- The `_decorator.rb` suffix is misleading, as that kind of overrides
has to be seen as monkey patches and not related to the decorator
pattern (see https://en.wikipedia.org/wiki/Decorator_pattern).

- Although it's something needed a lot of times, monkey patching core
classes should be seen as a last resort after ruling out other
strategies (like configuring a custom class for some behavior). As such,
we should not encourage it.

- It may conflict with the naming used by other gems, like Draper
(https://github.com/drapergem/draper).

For all of that, it's something that we should address in the docs.

This change is backward compatible, as the running code doesn't belong
to the Solidus engine but to the previously generated applications,
which will keep it after upgrading.

The deleted code is, in fact, intended to work with the classic
autoloader (removed on Rails 7). However, it still works because of
https://github.com/rails/rails/blob/296ef7a17221e81881e38b51aa2b014d7a28bac5/activesupport/lib/active_support/dependencies/require_dependency.rb,
which is deprecated.

On a related information, the recommended `app/overrides/` directory is
the same used by `deface` (https://github.com/spree/deface), a common
dependency in Solidus projects. However, both types of overrides can
coexist without interfering with each other (see
solidusio#3010 (comment)).
We decided to tackle the issue upstream on `deface` (see
solidusio#3010 (comment)).

Closes solidusio#3010

CHANGELOG.md

@@ -5,6 +5,40 @@
 
 - Fix CSRF forgery protection bypass for Spree::OrdersController#populate [GHSA-h3fg-h5v3-vf8m](https://github.com/solidusio/solidus/security/advisories/GHSA-h3fg-h5v3-vf8m)
 
+**Other important changes**
+
+New Solidus applications won't autoload files matching `app/**/*_decorator*.rb`
+pattern anymore. For previous Solidus applications, it's something that will
+keep working as the responsible code was added to your `config/application.rb`
+when Solidus was installed. That code is intended to work with Rails' classic
+autoloader, deprecated on Rails 6 and removed on Rails 7. It keeps working
+because of a [compatibility
+layer](https://github.com/rails/rails/blob/296ef7a17221e81881e38b51aa2b014d7a28bac5/activesupport/lib/active_support/dependencies/require_dependency.rb)
+which is also deprecated. However, it may be eventually removed, so you're
+better off updating your `application.rb`  file. You should substitute:
+
+```ruby
+config.to_prepare do
+  Dir.glob(Rails.root.join('app/**/*_decorator*.rb')) do |path|
+    require_dependency(path)
+  end
+```
+
+With:
+
+```ruby
+overrides = "#{Rails.root}/app/overrides" # use your actual directory here
+Rails.autoloaders.main.ignore(overrides)
+config.to_prepare do
+  Dir.glob("#{overrides}/**/*_decorator*.rb").each do |override|
+    load override
+  end
+end
+```
+
+You might also want to adopt the [current Rails recommendation but
+overrides](https://guides.rubyonrails.org/engines.html#improving-engine-functionality).
+
 ### Core
 
 - Add configuration option for `migration_path` [#4190](https://github.com/solidusio/solidus/pull/4190) ([SuperGoodSoft](https://github.com/supergoodsoft/))

core/lib/generators/solidus/install/install_generator.rb

@@ -103,17 +103,6 @@ def create_overrides_directory
     end
 
     def configure_application
-      application <<-RUBY
-    # Load application's model / class decorators
-    initializer 'spree.decorators' do |app|
-      config.to_prepare do
-        Dir.glob(Rails.root.join('app/**/*_decorator*.rb')) do |path|
-          require_dependency(path)
-        end
-      end
-    end
-      RUBY
-
       if !options[:enforce_available_locales].nil?
         application <<-RUBY
     # Prevent this deprecation message: https://github.com/svenfuchs/i18n/commit/3b6e56e