ISSUE-13340: PIP-122: Change loadBalancer default loadSheddingStrategy to ThresholdShedder

[sijie]

Original Issue: apache#13340

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Motivation

The ThresholdShedder load balance policy since Pulsar 2.6.0 by apache#6772. It can resolve many load balance issues of OverloadShedder and works well in many Pulsar production clusters.

In Pulsar 2.6.0, 2.7.0, 2.8.0 and 2.9.0, Pulsar's default load balance policy is OverloadShedder.

I think it's a good time for 2.10 to change default load balance policy to ThresholdShedder, it will make throughput more balance between brokers.

Proposed Changes

In 2.10 release，for broker.conf, change loadBalancerLoadSheddingStrategy from org.apache.pulsar.broker.loadbalance.impl.OverloadShedder to org.apache.pulsar.broker.loadbalance.impl.ThresholdShedder