Improves the error message shown in checkout when a saved payment method is invalid

[wjrosa]

Fixes STRIPE-572

Changes proposed in this Pull Request:

As requested in STRIPE-572, this PR improves the error message shown when a saved payment method becomes invalid. The goal is to reduce confusion among shoppers and merchants. Shoppers may then attempt to use another method when this happens.

Error preview:

Testing instructions

• Checkout to this branch on your test environment (fix/improving-error-message-for-detached-methods)
• Connect your Stripe account
• As a shopper, add a new payment method using the My Account screen
• As a merchant, go to your Stripe dashboard
• Open the shopper details
• Delete the newly added payment method
• As the same shopper, attempt to purchase using the same saved payment method
• Confirm the new error message above is shown

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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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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]

The change look ok and tests as described!

[daledupreez]

Over and above removing the polyfill, this PR is not working for me.

After debugging things somewhat extensively, it looks like we're missing code in WC_Stripe_UPE_Payment_Gateway->process_payment_with_saved_payment_method() to call $this->get_payment_intent_error_message() after we have determined that the error is not retryable and before we call $this->throw_localized_message() (where we don't have any intent-specific logic -- see below). I also checked, and we can't simply hand the modified error message to $this->throw_localized_message(), as it will get overridden based on the error code. 🙃

woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-upe-payment-gateway.php

Lines 1429 to 1438 in 7d055cd

	if ( ! empty( $intent->error ) ) {
	$this->maybe_remove_non_existent_customer( $intent->error, $order );
	// We want to retry (apparently).
	if ( $this->is_retryable_error( $intent->error ) ) {
	return $this->retry_after_error( $intent, $order, $can_retry );
	}
	$this->throw_localized_message( $intent, $order );
	}

I am also having a separate issue where the returned error message is not being displayed in the UI, but that is unrelated to this PR.

[daledupreez]

We don't need our own polyfill for this, as a polyfill for str_contains() has been in WordPress since 5.9.0 as per WordPress/wordpress-develop@3cc8f12. Given that 5.9.0 was released in early 2022 and our minimum supported version is 6.7, I think we should rely on the WordPress core polyfill.