GH-3706: Align RetryTopicConfigurationBuilder default with @RetryableTopic

[h2jinee]

Fixes #3706

Summary

Align the default sameIntervalTopicReuseStrategy in RetryTopicConfigurationBuilder with @RetryableTopic annotation (both now default to SINGLE_TOPIC).

Changes

• Change default from MULTIPLE_TOPICS to SINGLE_TOPIC in RetryTopicConfigurationBuilder
• Update topic-naming.adoc to reflect new default behavior
• Add migration guide in whats-new.adoc
• Update tests to explicitly use MULTIPLE_TOPICS where needed

Migration

To restore previous behavior:

RetryTopicConfigurationBuilder
    .newInstance()
    .sameIntervalTopicReuseStrategy(SameIntervalTopicReuseStrategy.MULTIPLE_TOPICS)
    .create(template);