cmd/utils: use preconfigured testnet flags instead of networkid (#21561)

* cmd/utils: use preconfigured testnet flags instead of networkid

* cmd/utils: shorter description
Co-authored-by: Martin Holst Swende <martin@swende.se>

* Update flags.go
Co-authored-by: Martin Holst Swende <martin@swende.se>

cmd/utils/flags.go

@@ -128,7 +128,7 @@ var (
 	}
 	NetworkIdFlag = cli.Uint64Flag{
 		Name:  "networkid",
-		Usage: "Network identifier (integer, 1=Frontier, 3=Ropsten, 4=Rinkeby, 5=Görli)",
+		Usage: "Explicitly set network id (integer)(For testnets: use --ropsten, --rinkeby, --goerli instead)",
 		Value: eth.DefaultConfig.NetworkId,
 	}
 	GoerliFlag = cli.BoolFlag{