User validation failed message update to give users better advice

What does this MR do and why?

Updates the user validation failed message on registration to improve the user experience, per https://gitlab.com/gitlab-org/modelops/anti-abuse/anti-abuse-updates/-/issues/14

Screenshots or screen recordings

Change; Email is not allowed for sign-up. Check with your administrator.

To: Please use your regular email address. Check with your administrator.

Screenshot of previous functionality:

How to set up and validate locally

1. Register with a banned email domain
2. Validate that the new message is shown

MR acceptance checklist

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