Journey Orchestration: Step Events: Deprecate fields and update description of fields(ACSS-34754)

## What are the schemas that are affected by the issue
/journeyOrchestration/stepEvents/journeyStepEventActionExecutionFieldsMixin.schema.json
/journeyOrchestration/stepEvents/journeyStepEventDataFetchFieldsMixin.schema.json
/journeyOrchestration/stepEvents/journeyStepEventCommonFieldsMixin.schema.json
/journeyOrchestration/stepEvents/journeyStepEventIdentityFieldsMixin.schema.json

## What are examples of products that are impacted by the issue
None(as only description is updated of all fields and some fields have been deprecated)

Start by opening the four affected schemas: journeyStepEventActionExecutionFieldsMixin.schema.json, journeyStepEventDataFetchFieldsMixin.schema.json, journeyStepEventCommonFieldsMixin.schema.json, and journeyStepEventIdentityFieldsMixin.schema.json. Review their field descriptions and deprecated fields, then verify that the requested descriptions and deprecation annotations are applied consistently across all four files.

Written by the indexing model from the issue text.