Remove `Spree::CreditCard` naming from legacy backend payment forms

Introduction

The implementation of a new payment method in Solidus Backend requires the addition of a dedicated source form to support a payment source creation/reuse.

Desired Behavior

A more generic integration approach (that maintains the logic of payment sources) is needed.

Many payment platforms currently support several types of payment methods.
So for the same Spree::PaymentMethod, we should be able to create payment sources of different types, such as cards, bank debits, bank redirects, bank transfers, etc.
For this reason, the partial source form view should be as generic as possible without specific references to the type of payment source.

Current Behavior

However, the current behavior in Solidus Backend is lacking in clearly marking these distinctions, with several references to the Spree::CreditCard payment source integration still present within views and controllers.

Some points related to what was mentioned:

• Sources passed inside the source form, which is called previous_cards
• Expected attribute in the Spree::Admin::PaymentsController called card dedicated to the credit card payment method: _gateway.html.erb source form where the card param is used

Additional notes

• This problem was experienced during the implementation of a source_form in solidus_stripe: https://github.com/solidusio/solidus_stripe/issues/280
• ℹ️ This doesn't create implementation problems but only nomenclature problems.
• All that said mainly refers to Stripe, but I think the same concept can be applied to different payment platforms.

Start with backend/app/views/spree/admin/payments/_form.html.erb, backend/app/controllers/spree/admin/payments_controller.rb, and backend/app/views/spree/admin/payments/source_forms/_gateway.html.erb, following the linked references. Review how previous_cards and card are passed into source forms, then update the naming so the integration is generic while preserving payment-source creation and reuse behavior. Confirm the legacy CreditCard-specific names are no longer required.