Deprecating and replacing PRBs classes

[wjrosa]

Changes proposed in this Pull Request:

In this cleaning-up PR, I am deprecating existing backend classes related to PRBs and moving the logic to new, ECE-named classes.

Testing instructions

• Checkout to this branch on your test environment (dev/deprecating-and-replace-prbs-classes)
• Connect your Stripe account
• Enable express checkout methods
• As a shopper, access the store using a public-facing URL
• Add a product to your cart
• Confirm you can complete a payment using the express methods

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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