collab: Look up Stripe prices with lookup keys (#29715)

This PR makes it so we look up Stripe prices via lookup keys instead of
passing in the price IDs as environment variables.

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -554,9 +554,6 @@ impl TestServer {
                 migrations_path: None,
                 seed_path: None,
                 stripe_api_key: None,
-                stripe_zed_pro_price_id: None,
-                stripe_zed_pro_trial_price_id: None,
-                stripe_zed_free_price_id: None,
                 supermaven_admin_api_key: None,
                 user_backfiller_github_access_token: None,
                 kinesis_region: None,