[VSA] Modify VSA to use Deployment Frequency API

Overview

CI/CD Analytics and VSA use slightly different definitions of "deployment frequency", but should use the same definition so users always see consistent numbers. Because ~"group::release" implemented a public-facing API for CI/CD Analytics, we should be using that.

Proposal

Group-level VSA should use the Deployment Frequency API to retrieve data for the "deployment frequency" statistic.