network: fix build, std.posix.shutdown is gone in Zig 0.16

The dead-peer-detection PR (#3018) landed without a rebase onto the
Zig 0.16 upgrade, so main no longer compiles:

    src/network/Network.zig:398:14: error: root source file struct 'posix' has no member named 'shutdown'

Use the sys/net.zig wrapper introduced by the 0.16 port instead.

src/network/Network.zig

@@ -395,7 +395,7 @@ fn dropCdp(self: *Network, link: *CdpLink, err: ?anyerror, opts: DropCdpOpts) vo
     self.cdp_dirty = true;
 
     if (opts.shutdown_socket) {
-        posix.shutdown(link.socket, .both) catch {};
+        sys_net.shutdown(link.socket, .both) catch {};
     }
 
     if (opts.notify) {