Add freeUserCapEnforced field to Group type

What does this MR do and why?

This adds the field freeUserCapEnforced to the Group type in GraphQl which indicates if the group has limited free users when on a free plan.

FE MR: !91248 (merged) Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/357257

Screenshots or screen recordings

How to set up and validate locally

1. export GITLAB_SIMULATE_SAAS=1
2. Have customerDot running and set up correctly to work with your GitLab instance
3. gdk start
4. Enable free_user_cap as feature flag globally, and set the gitlab instance to namespace plans

   bin/rails c
   Feature.enable(:free_user_cap)
   ApplicationSetting.first.update(check_namespace_plan: true)

5. Visit http://localhost:3000/-/graphql-explorer and run the query

    query {
      group(fullPath:"PATH_TO_FREE_GROUP") {
       id
       enforceFreeUserCap
      }
   }

MR acceptance checklist

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