Tweak a log message for mock-el (#8599)

```bash
$ lcli mock-el ....
...
...
Dec 15 11:52:06.002 INFO  Metrics HTTP server started                   listen_address: "127.0.0.1:8551"
...
```

The log message "Metrics HTTP server" was misleading, as the server is actually a Mock Execution Client that provides a JSON-RPC API for testing purposes, not a metrics server.


  


Co-Authored-By: ackintosh <sora.akatsuki@gmail.com>

Author: ackintosh

beacon_node/execution_layer/src/test_utils/mod.rs

@@ -754,7 +754,7 @@ pub fn serve<E: EthSpec>(
 
     info!(
         listen_address = listening_socket.to_string(),
-        "Metrics HTTP server started"
+        "Mock execution client started"
     );
 
     Ok((listening_socket, server))