[Payment card / Subscription] refactor ADD_DEBIT_CARD_FORM Onyx key to be agnostic of debit vs credit in subscription context

[blimpich]

Right now we're using the Onyx key ADD_DEBIT_CARD_FORM in the add-payment-card page for the new subscription page. This is a misnomer because the card can be either a debit or credit card. #43572 fixed this in the frontend, now we need to fix it in the backend.

Extra Context:
PR where it was introduced and discussion that initiated all this: #42771 (comment)
relevant slack thread: https://expensify.slack.com/archives/C036QM0SLJK/p1717606582669759
where we probably need to address it in web: https://github.com/Expensify/Web-Expensify/pull/42245/files#r1642211354
where App TODO is:

App/src/pages/settings/Subscription/PaymentCard/AddPaymentCard.tsx

Line 32 in c4482aa

// TODO refactor ONYXKEYS.FORMS.ADD_DEBIT_CARD_FORM to ONYXKEYS.FORMS.ADD_CARD_FORM as a follow up

Slack thread keeping track of Callstack engineer who is working on the frontend portion of this: https://expensify.slack.com/archives/C036QM0SLJK/p1718659002462059

[blimpich]

Waiting on #43572 to be merged I think before I can do this

[blimpich]

still waiting

[blimpich]

unblocked, will work on this today