Add @tkuah to CODEOWNERS

What does this MR do?

Adds @tkuah to CODEOWNERS file

• Maintainer MR - gitlab-com/www-gitlab-com!23069 (merged)

Closes gitlab-com/www-gitlab-com#3357 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers