Convert top_level_group_creation_enabled into an admin/configuration setting.

As part of an incident review gitlab-com/gl-infra/production-engineering#12801 (closed), we limited the ability for creation of top-level groups via API !56360 (merged).

The mechanism allows us to manage abuse risk on GitLab.com however is the default behaviour for GitLab SM instances. It is therefore default enabled but manually disabled for GitLab.com. In order to remove the feature flag for longer term, we should migrate this to a configuration setting (& to also avoid confusion where an engineer may enable the FF in GitLab.com production)