Make display_gitlab_credits_user_data configurable in admin area

What does this MR do and why?

This MR makes the new setting display_gitlab_credits_user_data configurable in admin area

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	!214508 (merged)
Make display_gitlab_credits_user_data configurable in admin area	👈 you're here
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

How to set up and validate locally

1. Go to https://gdk.test:3443/rails/features and enable usage_billing_dev
2. Navigate to Admin > Settings > General and expand Visibility and access controls
3. On GitLab Credits dashboard check/uncheck Display user data
4. Save changes
5. Compare with the value in rails console: Gitlab::CurrentSettings.display_gitlab_credits_user_data

MR acceptance checklist

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