Improve UI text of Visibility and access controls section

What does this MR do and why?

Improves UI text of the Admin Area settings - Visibility and access controls section - to comply with the GitLab Design System.

Resolves #350108 (comment 824304327).

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Log in to a GitLab instance as an administrator.
2. In the top bar, select Menu > Admin.
3. In the Settings section, select General.
4. Expand Visibility and access controls.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.