Remove 'GitLab' from user preferences header

What does this MR do?

Removes "Gitlab" from the user preferences header for navigation themes.
GitLab navigation theme to Navigation theme.

Are there points in the code the reviewer needs to double check?

No. HAML does not hold externalized strings, subject for a separate MR.

Why was this MR needed?

Wording consistency with following section headers.

Screenshots (if relevant)

Current UI for reference.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)