Change fulfillment_constriants Rust template to unstable

Author: joshua-arts

order-routing/rust/fulfillment-constraints/default/shopify.function.extension.toml.liquid

@@ -1,6 +1,6 @@
 name = "{{name}}"
 type = "{{extensionType}}"
-api_version = "2023-04"
+api_version = "unstable"
 
 [build]
 command = "cargo wasi build --release"