Make Matija a code owner

What does this Merge Request do?

As discussed and approved at gitlab-com/www-gitlab-com!75327 (merged), this MR adds @matteeyah as a *.rb code owner 🙂

I have also already granted @matteeyah maintainer access for the GDK:

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.