Update Journey Version Step Events XDM Mixin to add related to Objective Metric Id

## What are the schemas that are affected by the issue

https://github.com/adobe/xdm/blob/master/extensions/adobe/experience/journeyOrchestration/stepEvents/journeyStepEventJourneyFieldsMixin.schema.json

## What are examples of products that are impacted by the issue

Journey Orchestration, Customer Journey Management

Start with extensions/adobe/experience/journeyOrchestration/stepEvents/journeyStepEventJourneyFieldsMixin.schema.json and review its existing field definitions and conventions. Confirm the expected representation of the related Objective Metric ID, add it to the mixin, and verify that the schema remains valid and covers the requested Journey Orchestration and Customer Journey Management use cases.

Written by the indexing model from the issue text.