Add email to send to admin when a user is auto banned

What does this MR do and why?

This adds an email to alert an admin when a user is auto banned due to excessive repository downloads.

For use with the yet to be merged auto_ban_user_on_excessive_repo_downloads feature flag.

Issue: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/13

Screenshots or screen recordings

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How to set up and validate locally

Visit http://localhost:3000/rails/mailers/notify/user_auto_banned_email

MR acceptance checklist

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