set variant_processor to mini_magick

Seem like the default variant processor become vips.

https://github.com/openfoodfoundation/openfoodnetwork/actions/runs/8280341472/job/22656656944?pr=12209
rails/rails#44211

The comment on the source code doesn't seem to be updated
https://github.com/rails/rails/blob/4bb73233413f30fd7217bd7f08af44963f5832b1/activestorage/app/models/active_storage/variant.rb#L11

config/application.rb

@@ -249,6 +249,7 @@ module ::Reporting; end
     config.active_storage.content_types_to_serve_as_binary -= ["image/svg+xml"]
     config.active_storage.variable_content_types += ["image/svg+xml"]
     config.active_storage.url_options = config.action_controller.default_url_options
+    config.active_storage.variant_processor = :mini_magick
 
     config.exceptions_app = self.routes