Silence a ROR 7.1 deprecation warning by using Rails.cache as the default cache, if its available

[redterror]

This PR started from an investigation into a deprecation warning on a ROR 7.0 -> 7.1 upgrade. The specific error at the time was:

DEPRECATION WARNING: Support for `config.active_support.cache_format_version = 6.1` has been deprecated and will be removed in Rails 7.2.

Check the Rails upgrade guide at https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#new-activesupport-cache-serialization-format
for more information on how to upgrade.

This is triggered by this line in lib/warden/cognito.rb:

setting :cache, default: ActiveSupport::Cache::NullStore.new

Of course, nowhere here is cache_format_version explicitly set (nor should it be), but rather this is the default value in ActiveSupport.

• In the non-rails context, the warning is obvious - the default will change in an upcoming release.
• In a rails context this is confusing because it gets displayed even in the presence of a config.load_defaults 7.1 call in an app's config/application.rb file. I believe this is because the NullStore.new call gets executed at require time and thus before the Rails app config hooks start firing.

Workarounds:

• We could call ActiveSupport.cache_format_version = XX in warden-cognito's land, if Rails isn't defined. This seems bad because it might mess with an app's settings, and we don't really need to enforce a particular cache format here - that's not our job.
• We can change the default to be Rails.cache when Rails is defined.
• Other?

I opted for the 2nd option here since it seemed less intrusive and more in-line with what I'd expect in a Rails context. It also removed the deprecation in my original app.