VSD - Group merge request throughput should include subgroups

What does this MR do and why?

Enables includeSubgroups for the merge request throughput group metric on the Value stream dashboard.

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.

Screenshots or screen recordings

How to set up and validate locally

1. Visit the VSD for any group (ex. http://gdk.test:3000/groups/gitlab-org/-/analytics/dashboards/value_streams_dashboard)
2. Use the Network tab and find the groupMergeRequests GraphQL request
3. Verify that includeSubgroups: true is set

Related to #438293 (closed)