Allow matching only the repo-root for CODEOWNERS

What does this MR do?

When wanting to match files in the repo root, but only on the repo's root. Users would supply /* as the pattern.

In our old implementation we would match this pattern as *, but allowing * to match directory separators.

In this we treat the /* pattern as an entirely separate case: Matching all files in the root, but only one level deep.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8593

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• [-] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• [-] Security reports checked/validated by reviewer