Update UI text on Settings page

The GitLab Duo settings page has been updated without Tech Writer review, and some settings don't match our style guidelines. This issue is to review the page and correct some of the language.

Specifically:

• Ensure the terms match our word list ("GitLab Self-Managed" for example).
• Ensure we never use Duo without GitLab before it.
• Fix overall capitalization, punctuation, and style.

Ensure the docs match the UI text as well.

To access this page, go to the Admin area. On the left sidebar select GitLab Duo, then select Change configuration.

This work will require the GDK, but here is a screenshot: