Add `_` as allowed character in usernameRegexp

[ssgelm]

While disallowed from public github usernames, an underscore is a key part of github EMU usernames. See https://docs.github.com/en/enterprise-cloud@latest/admin/identity-and-access-management/understanding-iam-for-enterprises/about-enterprise-managed-users for more information.

[hmarr]

Thanks @ssgelm, makes sense 👍