Add display_gitlab_credits_user_data to app and namespace settings

What does this MR do and why?

This MR adds display_gitlab_credits_user_data to Application and Namespace settings.

This new setting is to support https://gitlab.com/gitlab-org/gitlab/-/issues/580868

MR Cadence

Description	MR
Add display_gitlab_credits_user_data to app and namespace settings	👈 you're here
Make display_gitlab_credits_user_data configurable in admin area	!214538 (merged)
Allow groups to update display_gitlab_credits_user_data	!215371
Hide user data when display_gitlab_credits_user_data is false	!215439

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

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