chore: fix misnamed CODEOWNERS file

What does this MR do?

According to https://docs.gitlab.com/ee/user/project/code_owners.html#set-up-code-owners this should be CODEOWNERS not CODEOWNERS.md

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10527
• 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