Change mentions of net2

[Darksonn]

The net2 crate is deprecated, so our documentation should recommend another crate. One such mention can be found here:

tokio/tokio/src/net/udp/socket.rs

Lines 168 to 170 in ce173fd

	/// This can be used in conjunction with net2's `UdpBuilder` interface to
	/// configure a socket before it's handed off, such as setting options like
	/// `reuse_address` or binding to multiple addresses.

I have not checked whether there are other mentions than the one above.

[zaharidichev]

I will take that