Only default to activestorage adapter if Rails version is supported

We only support the activestorage adapter with Rails 6.1 and above,
but we default to it, even if the Rails version is 6.0 and below.
Since we still support Rails 5.2 and 6.0, we cannot default to
the active storage adapter. The app won't boot unless we also
change the adapter in the spree initializer.

If the initializer, though, is nested in a `config.to_prepare` hook
(as recommended from Rails new autoloader zeitwerk [1]) the app
refuses to start anyhow, because the config is then taken from
the `app_configuration` defaults.

[1](https://guides.rubyonrails.org/autoloading_and_reloading_constants.html#use-case-1-during-boot-load-reloadable-code)

core/lib/spree/app_configuration.rb

@@ -497,7 +497,7 @@ def payment_canceller
     # @!attribute [rw] image_attachment_module
     # @return [Module] a module that can be included into Spree::Image to allow attachments
     # Enumerable of images adhering to the present_image_class interface
-    class_name_attribute :image_attachment_module, default: 'Spree::Image::ActiveStorageAttachment'
+    class_name_attribute :image_attachment_module, default: Rails.gem_version >= Gem::Version.new("6.1.0") ? "Spree::Image::ActiveStorageAttachment" : "Spree::Image::PaperclipAttachment"
 
     # @!attribute [rw] allowed_image_mime_types
     #
@@ -555,7 +555,7 @@ def payment_canceller
     # @!attribute [rw] taxon_attachment_module
     # @return [Module] a module that can be included into Spree::Taxon to allow attachments
     # Enumerable of taxons adhering to the present_taxon_class interface
-    class_name_attribute :taxon_attachment_module, default: 'Spree::Taxon::ActiveStorageAttachment'
+    class_name_attribute :taxon_attachment_module, default: Rails.gem_version >= Gem::Version.new("6.1.0") ? "Spree::Taxon::ActiveStorageAttachment" : "Spree::Taxon::PaperclipAttachment"
 
     # Configures the absolute path that contains the Solidus engine
     # migrations. This will be checked at app boot to confirm that all Solidus