Update Gitaly CODEOWNERS

What does this MR do and why?

• Removes the ee/lib/ee/gitlab/checks/ entry as the directory no longer exists.
• Adds lib/gitlab/gitaly_client.rb and lib/gitlab/gitaly_client/ as it contains the client code for Gitaly RPCs.
• Adds myself as a CODEOWNER as we'd like to start onboarding more Gitaly team members as Rails reviewers.

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