Pipeline to pipeline should default to PipelineBus V1

The recent pipeline to pipeline synchronisation improvement, [16194](https://github.com/elastic/logstash/pull/16194), it supposed have opt-in config `-Dlogstash.pipelinebus.implementation=v2` to let user chooses to use PipelineBus V2, however, the default value is set to "v2".

The issue names no specific file or test. Start by locating configuration handling for `logstash.pipelinebus.implementation`; done means omitting the option selects PipelineBus V1 while explicitly setting `v2` continues to select V2, with the relevant pipeline-to-pipeline synchronization tests passing.

Written by the indexing model from the issue text.