Marks a pricing plan as the primary pricing plan.
The makePlanPrimary() function returns a Promise that resolves to the now primary pricing plan.
Only a single plan can be marked as a primary plan at any given time. If there is an existing plan marked as primary, calling makePlanPrimary() causes the existing primary plan to lose its primary status.
When viewing pricing plans on the site, the primary plan is highlighted with a customizable ribbon.
This function requires elevated permissions and runs only on the backend and on dashboard pages.
ID of the pricing plan to set as the primary plan.