Make CODEOWNERS approvals required

What does this Merge Request do and why?

This MR proposes making CODEOWNERS approvals required for MRs to ensure the most relevant groups give their approval going forward:

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.