Use partitioned table on CommitStatus

What does this MR do and why?

ci_builds is now a partition of p_ci_builds and all queries targeting ci_builds can use p_ci_builds instead and the database will route them to the appropriate partition.

this MR changes the table name for the CI::Build models to use the routing table. This is under an env var feature flag because the code is evaluated at boot time and a feature flag would need to be accompanied by application restart to work.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #416455 (closed)