Extend Duo Core description for GitLab.com and Self-Managed/Dedicated

What does this MR do and why?

Extend Duo Core description for GitLab.com and Self-Managed/Dedicated following the request here.

References

• Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/550265

Screenshots or screen recordings

URL	Screenshot
Self-Managed http://gdk.test:8080/admin/gitlab_duo/configuration
GitLab.com http://gdk.test:8080/groups/duo-availablity-saas/-/settings/gitlab_duo/configuration

How to set up and validate locally

1. Get a Premium or Ultimate license for your Self-Managed instance
   1. Go to http://gdk.test:8080/admin/gitlab_duo/configuration and check the new description
2. Get a Premium or Ultimate subscription for a group of your SaaS instance
   1. Go to http://gdk.test:8080/groups//-/settings/gitlab_duo/configuration and check the new description

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.