collab: Make stripe_subscription_id and stripe_subscription_status nullable on billing_subscriptions (#36536)

This PR makes the `stripe_subscription_id` and
`stripe_subscription_status` columns nullable on the
`billing_subscriptions` table.

Release Notes:

- N/A

crates/collab/migrations/20250819225916_make_stripe_fields_optional_on_billing_subscription.sql

@@ -0,0 +1,3 @@
+alter table billing_subscriptions
+    alter column stripe_subscription_id drop not null,
+    alter column stripe_subscription_status drop not null;