allocator: decrease default io overload threshold

Previously, the default IO overload threshold for when stores are
considered overloaded was 0.8.

It is most often the case that IO overload will flap between (0.4,1.x).
It is desirable to not rebalance or allocate to these stores if there
are non-overloaded options.

This commit lowers the threshold to 0.5.

Part of: cockroachdb#96508

Release note: None

pkg/kv/kvserver/allocator/allocatorimpl/allocator_scorer.go

@@ -74,7 +74,7 @@ const (
 	// DefaultIOOverloadThreshold is used to avoid allocating to stores with an
 	// IO overload score greater than what's set. This is typically used in
 	// conjunction with IOOverloadMeanThreshold below.
-	DefaultIOOverloadThreshold = 0.8
+	DefaultIOOverloadThreshold = 0.5
 
 	// IOOverloadMeanThreshold is the percentage above the mean after which a
 	// store could be conisdered unhealthy if also exceeding the threshold.