PIP-238: Change the aggregatePublisherStatsByProducerName config default to true

### Motivation

The Pulsar community requested a PIP for this configuration default change.
https://github.com/apache/pulsar/pull/19114#pullrequestreview-1234028170

Context:
The index-based publisher stat aggregation(configured by aggregatePublisherStatsByProducerName=false, default, triggered by pulsar-admin topics partitioned-stats api) can wrongly aggregate publisher metrics if each partition stat returns a different size or order of the publisher stat list(from a large number of highly concurrent publishers). To avoid this issue, aggregatePublisherStatsByProducerName=true is the better default choice, as the state aggregation will be grouped by a unique key, producer names.

Discussion Thread:

https://lists.apache.org/thread/vofv1oz0wvzlwk4x9vk067rhkscn8bqo

### Goal

Change the aggregatePublisherStatsByProducerName config default to true

### API Changes

_No response_

### Implementation

PR: https://github.com/apache/pulsar/pull/19114

### Alternatives

N/A

### Anything else?

_No response_

Start by reviewing the aggregatePublisherStatsByProducerName configuration and the implementation PR linked in the issue. Confirm that the configuration default changes from false to true and that the resulting publisher-stat aggregation behavior matches the stated goal.

Written by the indexing model from the issue text.