Move default branch information

What does this MR do and why?

These are internal API endpoints, called by kas only. Kas does not currently use the value, so it's safe to move it. Kas will use the value once moved. Even without the value kas works fine, so it's a safe change in any case.

Relates to gitlab-org/cluster-integration/gitlab-agent#210 (closed).

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MR acceptance checklist

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