Legacy SEPA tokens using sources aren't being attached to the customer

[a-danae]

Describe the bug

When the Legacy checkout experience is enabled, SEPA tokens are created on the store, but they are not attached to the Stripe customer.

This works fine in 8.0.1.

To Reproduce

1. Select EUR as the store currency
2. Enable the Legacy checkout experience, At Stripe settings > Settings tab, /wp-admin/admin.php?page=wc-settings&tab=checkout&section=stripe&panel=settings
3. Enable SEPA under the Stripe payment methods. At Stripe settings > Payment methods tab /wp-admin/admin.php?page=wc-settings&tab=checkout&section=stripe&panel=methods
4. Log in as a shopper
5. Go to My account > Payment methods > Add payment method, at /my-account/add-payment-method/
6. Add a SEPA payment method
7. Notice a token is successfully created
8. Go to the Stripe dashboard > Customers > The customer associated with the shopper you just logged in. At https://dashboard.stripe.com/test/customers/<cus_xyz>
9. Notice there's no SEPA payment method attached

Expected behavior

When adding a token for a shopper, this should also be attached to the Stripe customer.