Split PE - Set the order's payment method ID to the processing payment method

[james-allan]

Fixes #2949

Changes proposed in this Pull Request:

This PR makes sure we set the order's payment method ID to the UPE payment method used to process the payment. Prior to our changes in add/deferred-intent this code use to set to stripe all the time (code link). In #2883, I changed it to set the orders payment ID to the retrievable payment method ID (ie SEPA for bancontact, iDEAL, Sofort). That PR was about subscription support so I suspect I was under the impression thats how it should work for orders, however that's not true.

Testing instructions

1. Enter Stripe credentials capable of processing iDEAL and Bancontact payments
2. In the Stripe advanced plugin settings, enable UPE.
3. Enable Bancontact, SEPA, Sofort (optional) and iDEAL payment methods.
4. Complete a purchase using Bancontact, iDEAL or Sofort.
5. View the order in WooCommerce > Orders.
   • On add/deferred-intent note the order's payment method is set to SEPA.
   • On this branch it should be set to the payment method you chose on checkout.

Before	After

I've tested subscription purchases and this still correctly sets the subscription's payment method SEPA and the parent order to Bancontact.

Order	Subscription
Order Payment Method is iDEAL	Subscription Payment Method is SEPA debit

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• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)

[a-danae]

Great catch here, @james-allan ! Thanks for this update.

✅ I was able to replicate the error in add/deferred-intent. SEPA was set as the used payment method for the order when other APMs were used, like Bancontact or iDEAL.
✅ In this branch, the APM that was used is correctly displayed for the order.