Add codeowners metric handler for archived projects

What does this MR do?

This MR fixes a bug with CODEOWNERS metric not being properly calculated for archived projects. Those projects have a repo, but have no default branch, so we need to fall back to 'HEAD' ref of the repo.

Related Sentry report: https://sentry.gitlab.net/gitlab/gitlabcom/issues/1693083/

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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Related to #326008 (closed)