op-node: Fix log level handler (#3689)

The handler returned from `log.LvlFilterHandler` wasn't being consumed anywhere, so log level configuration was being ignored.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

op-node/node/log.go

@@ -44,7 +44,7 @@ func (cfg *LogConfig) Check() error {
 func (cfg *LogConfig) NewLogger() log.Logger {
 	handler := log.StreamHandler(os.Stdout, format(cfg.Format, cfg.Color))
 	handler = log.SyncHandler(handler)
-	log.LvlFilterHandler(level(cfg.Level), handler)
+	handler = log.LvlFilterHandler(level(cfg.Level), handler)
 	logger := log.New()
 	logger.SetHandler(handler)
 	return logger