Update text on SM instance toggle

GitLab code suggestions is now powered by a 3P service provider. We should update the language on the self-managed instance toggles accordingly.

UX has drafted a proposal with updated text: https://gitlab.com/gitlab-org/gitlab/-/issues/413290/designs/self_managed.png

Note: UX mock-up still needs to be updated with latest feedback.

Our goal is to deliver this in 16.2

In scope:

• Update the text content on instance admin settings page to reflect latest UX design, legal language, and technical writing guidelines

Out of scope:

• No changes to behaviour are expected
• No new toggles are required