Bump version to 4.2.1

Author: brocaar

Cargo.lock

@@ -4,7 +4,7 @@
   repository = "https://github.com/chirpstack/chirpstack-mqtt-forwarder"
   homepage = "https://www.chirpstack.io/"
   license = "MIT"
-  version = "4.2.0"
+  version = "4.2.1"
   authors = ["Orne Brocaar <[email protected]>"]
   edition = "2021"
   publish = false