Duplicate wc_ajax_wcpay_add_to_cart endpoint is being overriden by WooPay

[gpressutto5]

Describe the bug

WooPay button's wc_ajax_wcpay_add_to_cart endpoint is taking precedence and overriding the endpoint that was meant to be used by Apple Pay. Currently this is not causing any bugs because both methods are identical, but it is confusing. These endpoints should be merged or one of them should be renamed to avoid conflicts.

To Reproduce

1. Enable WooPay in your store.
2. Add throw new \Exception('test'); as the first line of WC_Payments_Payment_Request_Button_Handler::ajax_add_to_cart().
3. Check the network tab of your browser's devtools, use the Apple Pay button and notice the call to wc_ajax_wcpay_add_to_cart was succesfull.
4. Add the same exception to WC_Payments_WooPay_Button_Handler::ajax_add_to_cart().
5. Repeat step 3 and notice the request fails.

[RadoslavGeorgiev]

Might be related to #7588.

This issue impacts "Additional Payment Methods (including Apple Pay / Google Pay)", so assigning to Heisenberg (based on team responsibilities Pc2DNy-3z-p2) cc team lead @frosso, similar to the related issue.

Assigning as part of Gamma Triage process PcreKM-yM-p2.