Rename CreateCrossProjectPipelineWorker to CreateDownstreamPipelineWorker

Problem

To create parent child pipelines we renamed CreateCrossProjectPipelineService to CreateDownstreamPipelineService. We should rename the worker to CreateDownstreamPipelineWorker.

Steps:

1. Create a new duplicate worker Ci::CreateDownstreamPipelineWorker and use it with a replacement of Ci::CreateCrossProjectPipelineWorker behind a feature flag as suggested in the doc.
2. Remove the feature flag in the next milestone.
3. Remove the worker in the subsequent milestone - https://docs.gitlab.com/ee/development/sidekiq_style_guide.html#removing-workers