Invoice.subscription is already moved to line item

[cosmeowpawlitan]

stripe-firebase-extensions/firestore-stripe-payments/functions/src/utils.ts

Line 324 in 96df628

.doc(invoice.subscription as string)

Although they do not metion anything about removal of the subscription field of the Invoice API object in the Change Log
, the removal is reflected on an update on the type file that the field become a nullable field.

The subscription id are now avaliable in lines.data.parent.subscription_item_details.subscription.

This change make invoice and subscription a m:n relationship.