feat: Default to least-in-flight balancing for Bigtable unary clients (#13802)

Author: neilw4

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/gaxx/grpc/BigtableChannelPoolSettings.java

@@ -60,7 +60,7 @@ public abstract class BigtableChannelPoolSettings {
 
   /** Load balancing strategy to use if environment variable is unset or invalid. */
   private static final LoadBalancingStrategy DEFAULT_LOAD_BALANCING_STRATEGY =
-      LoadBalancingStrategy.ROUND_ROBIN;
+      LoadBalancingStrategy.LEAST_IN_FLIGHT;
 
   /** Supported load-balancing strategies. */
   public enum LoadBalancingStrategy {