Setting Stripe Checkout as default payment flow, remove Update Billing Flow option

[dparker1005]

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Changes proposed in this Pull Request:

The Update Billing Flow option will now always match the Payment Flow option (ie offsite payments is offsite billing updates, onsite payments is onsite billing updates).

The new default will only affect new PMPro sites. Existing sites will retain their selected option.

Other information:

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Changelog entry

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