fix understated ipnet version requirement (#521)

`webrtc-util` uses `IpNet::with_netmask`, which was introduced in
version 2.6.0. If someone is using a `Cargo.lock` with an older version of
`ipnet`, they'll get a compile error when introducing `webrtc-util` (or
upgrading it to after this call was added). Adding the correct version
requirement in `Cargo.toml` forces the upgrade to fix this.

Author: scottlamb

util/Cargo.toml

@@ -22,7 +22,7 @@ sync = []
 tokio = { version = "1.32.0", features = ["full"] }
 lazy_static = "1"
 async-trait = "0.1"
-ipnet = "2"
+ipnet = "2.6.0"
 log = "0.4"
 rand = "0.8"
 bytes = "1"