Enforce free user top-level group limit in SaaS

What does this MR do and why?

Limit the number of top-level groups that can be created by free users on GitLab.com to 3 per account.

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/581811

Screenshots or screen recordings

How to set up and validate locally

1. Setup GDK locally simulating SAAS (i.e. add the below to your gdk.yml and gdk restart)

   env:
     GITLAB_SIMULATE_SAAS: "1"

2. Enable the feature flag in gdk rails c

   Feature.enable(:enforce_top_level_group_limits)

3. Create a new user and create 3 top-level groups
4. Try to create another top-level group and verify you receive an error message

MR acceptance checklist

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