Ignore plain-text columns from Users::CreditCardValidation model

What does this MR do and why?

• Resolves sub-task 3 of https://gitlab.com/gitlab-org/gitlab/-/issues/413525
• Follows up from !131365 (merged)
• This MR ignores the plain-text columns in the Users::CreditCardValidation model. In the previous MR we had removed all usage of the plain-text columns.

How to set up and validate locally

1. Run a sanity check by creating a new Users::CreditCardValidation record

> Users::CreditCardValidation.create(user_id: User.first.id, last_digits: 1111, credit_card_validated_at: Date.today, expiration_date: 1.year.from_now.to_date, holder_name: 'John Doe', network: 'Visa')

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #413525