Check if columns exist before running credit card hashing background migration

What does this MR do and why?

• Solves #473713 (closed) & omnibus-gitlab#8653 (closed). The finalise migration was introduced in 17.2, !157836 (merged) which tried to finalise the ConvertCreditCardValidationDataToHashes background migration, but the plain-text columns, such as last_digits were already dropped in 16.8.
• This fix just checks if those columns exist before running the background migration

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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