Re-enabling migration of PRBs settings to ECE

[wjrosa]

Towards STRIPE-724

Changes proposed in this Pull Request:

As part of the effort to remove all code related to Payment Request Buttons, this PR migrates all settings to the new format, with the express_checkout preffix. It also adapts all code to use the new format, and the unit tests.

Testing instructions

• Create a new JN website
• Install WC and the Stripe extension (any version, but not this one)
• Connect your Stripe account
• Enable express checkout buttons
• Build this branch (update/re-enabling-migration-of-settings-to-ece)
• Install it in the same store
• Confirm ECE is still working (both settings and checkout)
• Take a look on your database settings (woocommerce_stripe_settings) and confirm all ECE keys were introduced (with the express_checkout prefix)

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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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