fix: use user email for esp sync purposes

[leogermani]

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• Does your code follow the WordPress' coding standards and VIP Go coding standards?
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Changes proposed in this Pull Request:

When a user purchases something on a site with RAS activated and its info gets synced to the ESP, we were prioritizing the billing email over the user registered email.

This means that if the user registers with one email, but then uses a different email to make a purchase, it would create a new user in the ESP - but not in WP.

This PR makes sure we always use the user email for sync purposes.

Was there any specific reasons why we wanted to use billing email there? Do we really want this change?

How to test the changes in this Pull Request:

1. on trunk
2. Register with a user
3. Make a donation or purchase something, but change your email in the checkout form
4. Confirm ESP syncs info about this purchase using the modified email
5. On this branch, confirm that all ESP syncs are tied to the original user email

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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[adekbadek]

Was there any specific reasons why we wanted to use billing email there? Do we really want this change?

I don't think it was intentional, it was rather assumed that the billing email will be the same as the user email.

[github-actions]

Hey @leogermani, good job getting this PR merged! 🎉

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