collab: Record cancellation reason on billing subscriptions (#22853)

This PR updates the `billing_subscriptions` in the database to record
the cancellation reason from Stripe.

We're primarily interested in this so we can check for subscriptions
that were canceled for being `past_due`.

Release Notes:

- N/A

crates/collab/migrations/20250108184547_add_stripe_cancellation_reason_to_billing_subscriptions.sql

@@ -0,0 +1,2 @@
+alter table billing_subscriptions
+add column stripe_cancellation_reason text;