collab: Add POST /users/:id/update_plan endpoint (#34953)

This PR adds a new `POST /users/:id/update_plan` endpoint to Collab to
allow Cloud to push down plan updates to users.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -785,7 +785,7 @@ async fn handle_customer_subscription_event(
 
     // When the user's subscription changes, push down any changes to their plan.
     rpc_server
-        .update_plan_for_user(billing_customer.user_id)
+        .update_plan_for_user_legacy(billing_customer.user_id)
         .await
         .trace_err();