Fix incorrect CODEOWNERS syntax

What does this MR do?

The new CODEOWNERS file is of the wrong format. This MR updates the Maintainers block to be a single line.

What are the relevant issue numbers?

• gitlab-org/secure/general#52 (closed)
• gitlab-com/www-gitlab-com#11607 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer