Fix payment method detachment in live mode

[wjrosa]

Fixes #3582

Changes proposed in this Pull Request:

This PR fixes some issues that may happen with staging sites detaching payment methods from production accounts. Basically, some additional checks were added before detaching a payment method (with the new checkout experience enabled).

Testing instructions

1. Create a WooCommerce Subscription
2. Subscribe to a service with a test user
3. Login to the dashboard.stripe.com and confirm that the Customer was created and has a Payment Method in place
4. Setup a clone of that site, e.g. with WP Staging.
5. Check if Stripe is connected on the clone. It might have to be re-connected given the new Stripe connection flow/mechanism.
6. From WP Admin panel of the clone site, go to Users and delete the customer you created on Step 2.
7. Go back to the Stripe Dashboard and look for the Customer. On this branch (fix/pm-dettachment-in-live-mode) the payment method will not be detached

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• [ ] Covered with tests (or have a good reason not to test in description ☝️)
• [x] 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)
• [ ] Added the needs docs label (or does not apply)
• [ ] Included this PR in the Release Thread scope (or does not apply)

cc @grig-23