Enable in-product emails only for free instances [RUN AS-IF-FOSS]

What does this MR do?

Only send the product marketing emails on non-paid instances.

Issue: gitlab-org/growth/team-tasks#315 (closed) This MR only targets self managed for now. Only targeting free namespaces on .com will be done in a second step.

Why? From Issue: gitlab-org/growth/team-tasks#315 (closed)

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

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team