Update High Traffic table CI list

What does this MR do and why?

This MR updates our High Traffic tables CI list with new tables > 10 GB.

I've used the following command on a thin clone to get those numbers 👇

SELECT pg_size_pretty( pg_total_relation_size(TABLE_NAME));

table name	size
ci_job_artifact_states	43 GB
ci_job_variables	19 GB
ci_namespace_mirrors	12 GB
ci_refs	11 GB

MR acceptance checklist

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