Up minimum socket2 to 0.6.0

Now that hyper-util uses `socket2::Socket::set_tcp_nodelay`, which was
added in [0.6.0]. It used to be called `socket2::Socket::set_nodelay` in
[0.5.10].

[0.5.10]: https://docs.rs/socket2/0.5.10/socket2/struct.Socket.html#method.set_nodelay
[0.6.0]: https://docs.rs/socket2/0.6.0/socket2/struct.Socket.html#method.set_tcp_nodelay

Author: erickt

Cargo.toml

@@ -29,7 +29,7 @@ ipnet = { version = "2.9", optional = true }
 libc = { version = "0.2", optional = true }
 percent-encoding = { version = "2.3", optional = true }
 pin-project-lite = "0.2.4"
-socket2 = { version = ">=0.5.9, <0.7", optional = true, features = ["all"] }
+socket2 = { version = ">=0.6.0, <0.7", optional = true, features = ["all"] }
 tracing = { version = "0.1", default-features = false, features = ["std"], optional = true }
 tokio = { version = "1", optional = true, default-features = false  }
 tower-layer = { version = "0.3", optional = true }