Disable Mplex by default (#9365)

Co-Authored-By: João Oliveira <hello@jxs.pt>

beacon_node/src/cli.rs

@@ -387,6 +387,14 @@ pub fn cli_app() -> Command {
                 .help("Disables the quic transport. The node will rely solely on the TCP transport for libp2p connections.")
                 .display_order(0)
         )
+        .arg(
+            Arg::new("enable-mplex")
+                .long("enable-mplex")
+                .action(ArgAction::SetTrue)
+                .help_heading(FLAG_HEADER)
+                .help("Enables mplex multiplexer alongside yamux. Yamux is preferred when both are available.")
+                .display_order(0)
+        )
         .arg(
             Arg::new("disable-peer-scoring")
                 .long("disable-peer-scoring")