New purchase flow should disallow payments for subgroups

Problem

The new checkout flow shows subgroups in the group selection list for payment.

• But we don't indeed allow subgroups to have subscriptions
• For example, the experiment with purchase flow for free payments links existing top level groups to the new checkout flow. Actually the top level group comes pre-selected. However it's possible for some user to modify their selection and choose a subgroup.

Resolution Suggestion

Only top-level groups must be shown in the payment group selection list.

Followup check

Checked and saw no recent subgroups with paid subscriptions using this flow. So we didn't have an issue so far.

There are 26 subgroups on a paid plan, but they are rather old.

cc @jstava @amandarueda @s_awezec for prioritization

cc @gitlab-org/growth/conversion