chore: bump version

Release version 0.3.0

Main features:
- persist last IP checked to ensure DNS records can be served upon
  restart
- end-to-end testing of the NixOS module ensures most changes will not
  result in a broken DNS server

Cargo.lock

@@ -1162,7 +1162,7 @@ checksum = "9c8d87e72b64a3b4db28d11ce29237c246188f4f51057d65a7eab63b7987e423"
 
 [[package]]
 name = "webnsupdate"
-version = "0.3.0-dev"
+version = "0.3.0"
 dependencies = [
  "axum",
  "axum-auth",