Limit value streams max size to prevent timeouts

What does this MR do and why?

Limit value streams max size for groups from 100 (default) to 20 to prevent timeouts

Changelog: changed

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

1. Create 21 value streams under a group. They can be default template, and call them stream1...stream21.
2. With the query below, you should only see the first 20 value streams.

With a cold cache on gitlab.com, try:

{
  group(fullPath: "gitlab-org") {
    valueStreams(first: 20) {
      nodes {
        name
        stages {
          name
          metrics(timeframe: { start: "2024-08-01", end: "2024-08-30"}) {
            items {
              nodes {
                duration
                endEventTimestamp
                record {
                  ... on Issue {
                    id
                    title
                  }
                  ... on MergeRequest {
                    id
                    title
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

and it should be OK.

Related to #480705 (closed)