Fix inconsistent disabled states in the credit card form

[matt-west]

Where

/me/purchases/add-payment-method

Problem

When saving a new payment method we disable the fields in the credit card form, but the disabled states are very inconsistent.

Solution

• Display consistent disabled states for all fields in the credit card form.

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] WPCOM Store & Purchases: The issue relates to the purchasing process, specifically the credit card form used for payment methods.
• [Feature] Checkout: The problem is specifically about the checkout process when saving a new payment method, which involves the credit card form.
• [Feature] Customer Seller Experience: The inconsistency in disabled states affects the overall experience of customers when entering payment information.