Fix bug with deployment score calculation

What does this MR do and why?

Fix bug with deployment score calculation

Deployment score should be based on deployment frequency, not total deployments count.

Full fix requires background migration of existing data. I'll provide it in a separate MR for simplicity.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Run example GraphQL query:

query {
  group(fullPath: "gitlab-org") {
    doraPerformanceScoreCounts {
      nodes {
        metricName
        highProjectsCount
        mediumProjectsCount
        lowProjectsCount
      }
    }
  }
}

Related to #471741 (closed)