Update chirpstack_api crate.

This update extends the gateway stats with duty-cycle metrics (if
provided by the gateway).

Author: brocaar

Cargo.lock

@@ -16,7 +16,7 @@
     "usage",
     "derive",
   ] }
-  chirpstack_api = { version = "4.7", default-features = false, features = [
+  chirpstack_api = { version = "4.8.0-test.2", default-features = false, features = [
     "json",
   ] }
   lrwn_filters = { version = "4.7", features = ["serde"] }