Marks a pricing plan as the primary pricing plan.
The makePlanPrimary() function returns a Promise that resolves to the
pricing plan that is now the primary one.
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 status.
When viewing pricing plans on the site, the primary plan is highlighted with a customizable ribbon.
Only users with "Manage Pricing Plans" permissions can mark a plan as a primary plan.
ID of the plan to be set as the primary plan.