cmd/utils: point users to --syncmode under DEPRECATED (#16572)

Indicate that --light and --fast options are replaced by --syncmode

cmd/utils/flags.go

@@ -158,11 +158,11 @@ var (
 	}
 	FastSyncFlag = cli.BoolFlag{
 		Name:  "fast",
-		Usage: "Enable fast syncing through state downloads",
+		Usage: "Enable fast syncing through state downloads (replaced by --syncmode)",
 	}
 	LightModeFlag = cli.BoolFlag{
 		Name:  "light",
-		Usage: "Enable light client mode",
+		Usage: "Enable light client mode (replaced by --syncmode)",
 	}
 	defaultSyncMode = eth.DefaultConfig.SyncMode
 	SyncModeFlag    = TextMarshalerFlag{