Deprecate and remove Legacy Checkout instances

[wjrosa]

Fixes STRIPE-524

Changes proposed in this Pull Request:

This PR removes some unnecessary instantiations of the legacy checkout, as UPE is now the only option available. I am also deprecating the main legacy gateway class to make it easier to remove other references to it later.

Testing instructions

• Checkout to this branch on your test environment (update/deprecate-and-remove-legacy-checkout-instances)
• Rebuild your test store (or create a new one using something like JN)
• Install the extension
• Connect your Stripe account
• Confirm the Stripe settings page is working fine
• Check your settings database key and confirm upe_checkout_experience_enabled is set to yes

---

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

Changelog entry

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Changelog Entry Comment

Comment

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)

[diegocurbelo]

This looks good. I could not find any other instantiation of the Legacy gateway.

I think we should also update the test suite accordingly. It's still being used in these 4 test files:

• WC_Stripe_Payment_Gateway_Test.php
• WC_REST_Stripe_Settings_Controller_Test.php
• WC_Stripe_Settings_Controller_Test.php
• WC_Stripe_Express_Checkout_Helper_Test.php

[wjrosa]

Thanks for the review, Diego! I removed all references from the unit tests in df553ae

[daledupreez]

Changes LGTM! I have some feedback/commentary, but none of it requires immediate action.

[daledupreez]

Non-blocking style note: I prefer using the fully-qualified class name so it's easier to see exactly which class is in use, especially in diffs. I think we should discuss this as a team before making changes to any code, though.

Suggested change

	* @var WC_Stripe_UPE_Payment_Gateway
	* @var \WC_Stripe_UPE_Payment_Gateway