build(deps): update netlink-packet-route requirement

Updates the requirements on [netlink-packet-route](https://github.com/rust-netlink/netlink-packet-route) to permit the latest version.
- [Release notes](https://github.com/rust-netlink/netlink-packet-route/releases)
- [Changelog](https://github.com/rust-netlink/netlink-packet-route/blob/main/CHANGELOG)
- [Commits](rust-netlink/netlink-packet-route@v0.19.0...v0.21.0)

---
updated-dependencies:
- dependency-name: netlink-packet-route
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

rattan-core/Cargo.toml

@@ -23,7 +23,7 @@ ipnet = { version = "2.8.0", features = ["serde"] }
 lazy_static = "1.4.0"
 libc = "0.2.141"
 netem-trace = { workspace = true }
-netlink-packet-route = "0.19.0"
+netlink-packet-route = "0.21.0"
 nix = { workspace = true }
 once_cell = { workspace = true }
 parking_lot = "0.12.1"