kvserver: don't log "local QPS is below max threshold" by default

This log message is the 9th most voluminous across the CC fleet.
It's not surprising since most stores are idle.
There's no good reason to have it logged by default - it doesn't say
anything interesting.
Let's just keep it in traces.

Release note: None

pkg/kv/kvserver/store_rebalancer.go

@@ -294,7 +294,7 @@ func (sr *StoreRebalancer) rebalanceStore(
 	// cluster-level overfull threshold of QPS.
 	qpsMaxThreshold := overfullQPSThreshold(options, allStoresList.candidateQueriesPerSecond.mean)
 	if !(localDesc.Capacity.QueriesPerSecond > qpsMaxThreshold) {
-		log.Infof(ctx, "local QPS %.2f is below max threshold %.2f (mean=%.2f); no rebalancing needed",
+		log.VEventf(ctx, 1, "local QPS %.2f is below max threshold %.2f (mean=%.2f); no rebalancing needed",
 			localDesc.Capacity.QueriesPerSecond, qpsMaxThreshold, allStoresList.candidateQueriesPerSecond.mean)
 		return
 	}