Fix setup instructions for Rails 7

[diegomichel]

Summary

Since Rails 7, it's no longer possible to reference a loadable constant on the initialize phase [1]. We need to wrap the setup within a to_prepare block, as it's already documented in our guides.

Credits to: @waiting-for-dev

Pretty much a copy of: solidusio/solidus_paypal_braintree#332

Checklist

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[waiting-for-dev]

Thanks!