Review .gitlab/CODEOWNERS entries

What does this MR do?

This MR fixes existing or removes orphaned entries in .gitlab/CODEOWNERS as spit out by https://gitlab.com/-/snippets/2085053.

Refs #216066 (comment 521068166)

Screenshots (strongly suggested)

n/a

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • I have not included a changelog entry because it's not a user-facing change.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content