config action dispatch: migrate cookie serializer from :marshal to …

…`:json`

GitHub: ref rangubaGH-7

In Rails v7, this setting defaults to `:json`.

Using `:marshal` as the default value was for backward compatibility with Rails v4.0.
We no longer need this default because we have been using the JSON format by overwriting it in `config/initializers/cookies_serializer.rb`.

In Ranguba, this change has no effect since we don't use cookies.

ref: rails/rails#42538
ref: https://guides.rubyonrails.org/configuring.html#config-action-dispatch-cookies-serializer

config/initializers/new_framework_defaults_7_0.rb

@@ -115,7 +115,7 @@
 # If you're upgrading and haven't set `cookies_serializer` previously, your cookie serializer
 # is `:marshal`. The default for new apps is `:json`.
 #
-# Rails.application.config.action_dispatch.cookies_serializer = :json
+Rails.application.config.action_dispatch.cookies_serializer = :json
 #
 #
 # To migrate an existing application to the `:json` serializer, use the `:hybrid` option.