Simplify the steps to make changes to config.json (#5186)

This change introduces following simplified steps to follow 
during config migration.

```
 // Steps to move from version N to version N+1
 // 1. Add new struct serverConfigVN+1 in config-versions.go
 // 2. Set configCurrentVersion to "N+1"
 // 3. Set serverConfigCurrent to serverConfigVN+1
 // 4. Add new migration function (ex. func migrateVNToVN+1()) in config-migrate.go
 // 5. Call migrateVNToVN+1() from migrateConfig() in config-migrate.go
 // 6. Make changes in config-current_test.go for any test change
```

cmd/config-migrate.go

@@ -1500,7 +1500,7 @@ func migrateV19ToV20() error {
 	}
 
 	// Copy over fields from V19 into V20 config struct
-	srvConfig := &serverConfigV20{
+	srvConfig := &serverConfig{
 		Logger: &loggers{},
 		Notify: &notifier{},
 	}