collab: Don't require payment method to subscribe to Zed Free (#30398)

This PR makes it so we don't require a payment method to subscribe to
the Zed Free plan.

Release Notes:

- N/A

crates/collab/src/stripe_billing.rs

@@ -248,6 +248,8 @@ impl StripeBilling {
 
         let mut params = stripe::CreateCheckoutSession::new();
         params.mode = Some(stripe::CheckoutSessionMode::Subscription);
+        params.payment_method_collection =
+            Some(stripe::CheckoutSessionPaymentMethodCollection::IfRequired);
         params.customer = Some(customer_id);
         params.client_reference_id = Some(github_login);
         params.line_items = Some(vec![stripe::CreateCheckoutSessionLineItems {