[Split PE] Duplicated saved APM tokens when coming from non-PE

[a-danae]

Describe the bug

When saving APMs in develop and switching to Split PE, they get duplicated. This would be the scenario when users upgrade to 8.0.0.

We need to handle this migration gracefully:

• The tokens must not appear as duplicated for the shopper in 8.0.0.
• The old tokens must not be removed from the database, so merchants could roll back to < 8.0.0 if needed without the shoppers losing their saved payment methods.

To Reproduce

Creating the tokens in an older version

1. Check out develop
2. Select EUR as the store currency
3. Under the Stripe settings, enable UPE and enable SEPA
4. As a shopper, go to the checkout page
5. Select "Use a new payment method"
6. Select SEPA
7. Enter AT611904300234573201 and check off "Save payment information to my account for future purchases"
8. Place the order
9. Go to My Account > Payment methods
10. Notice there's a token for the SEPA payment method, all good

Upgrading to 8.0.0

1. Check out add/deferred-intent
2. Reload the My Account > Payment methods page
3. Notice that the tokens for SEPA got duplicated
4. Go to the shortcode checkout page
5. Notice the SEPA payment methods exist under both the Credit Card and the SEPA gateways

Expected behavior The APMs must not appear duplicated for the merchant or the shopper.

Additional context

Butwhy

• When retrieving the tokens for a user, we check them with the payment methods saved in Stripe. The payment methods in Stripe are our source of truth. Local payment methods will get created or deleted according to those. This is handled by WC_Stripe_Payment_Tokens::woocommerce_get_customer_upe_payment_tokens().
• In previous versions, all APMs were saved under the stripe gateway. Since 8.0.0, each APM has its own gateway instead.
• The APM tokens saved in previous versions will belong to the stripe gateway.
• When retrieving the SEPA payment methods from Stripe, 8.0.0 we'll expect them to exist under the stripe_sepa_debit gateway. We'll create them because these wouldn't exist, thus the duplication.

[a-danae]

@wjrosa I was working on another issue and ended up fixing this behavior as a side-effect of those changes.

This should be fixed by https://github.com/woocommerce/woocommerce-gateway-stripe/pull/2926