Only send in-product emails to free groups

What does this MR do?

Issue: gitlab-org/growth/team-tasks#373 (closed)

Customer success sends many helpful emails to admins of paid accounts to help them get started with their namespace / instance.

This changes the selection of groups eligible for in-product-marketing emails to only free groups (non-paid plan or on a trial).

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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