[Next major] Set the latest API version as the default

[richardm-stripe]

Analog to this change in stripe-node.

Changelog

⚠️ ACTION REQUIRED: the breaking change in this release likely affects you ⚠️

In this release, Stripe API Version 2023-08-16 (the latest at time of release) will be sent by default on all requests.
The previous default was to use your Stripe account's default API version.

To successfully upgrade to stripe-ruby v9, you must either

1. (Recommended) Upgrade your integration to be compatible with API Version 2023-08-16.

   Please read the API Changelog carefully for each API Version from 2023-08-16 back to your Stripe account's default API version. Determine if you are using any of the APIs that have changed in a breaking way, and adjust your integration accordingly. Carefully test your changes with Stripe Test Mode before deploying them to production.

   You can read the v9 migration guide for more detailed instructions.

2. (Alternative option) Specify a version other than 2023-08-16 when initializing stripe-ruby.

   If you were previously initializing stripe-ruby without an explicit API Version, you can postpone modifying your integration by specifying a version equal to your Stripe account's default API version. For example:

     require 'stripe'
     Stripe.api_key = "sk_test_..."
   + Stripe.api_version = '2020-08-27'

   If you were already initializing stripe-ruby with an explicit API Version, upgrading to v9 will not affect your integration.

   Read the v9 migration guide for more details.

Going forward, each major release of this library will be pinned by default to the latest Stripe API Version at the time of release.

That is, instead of upgrading stripe-ruby and separately upgrading your Stripe API Version through the Stripe Dashboard. whenever you upgrade major versions of stripe-ruby, you should also upgrade your integration to be compatible with the latest Stripe API version.