Change p2p default port to 8084 (#21162)

## Description 

8084 is mentioned to be the default p2p port in docs:
https://docs.sui.io/guides/operator/validator-tasks#connectivity
And validator / transaction RPC port defaults to 8080 already.
So changing the default p2p port from 8080 to 8084 in `P2pConfig`.
I believe the change is backward compatible but lmk otherwise.

## Test plan 

CI

---

## Release notes

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- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] gRPC:
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:

crates/sui-config/src/p2p.rs

@@ -40,7 +40,7 @@ pub struct P2pConfig {
 }
 
 fn default_listen_address() -> SocketAddr {
-    "0.0.0.0:8080".parse().unwrap()
+    "0.0.0.0:8084".parse().unwrap()
 }
 
 impl Default for P2pConfig {