Refactor Workflow Tests and Services to Use System.Text.Json

- Updated WorkflowTest.cs to replace Newtonsoft.Json with System.Text.Json for JSON handling.
- Modified assertions in tests to utilize OpenJsonObjectResponse instead of OpenJObjectResponse.
- Changed BulkWorkflowUpdateServiceTest to use System.Text.Json for serialization settings.
- Refactored Workflow.cs and WorkflowModel.cs to remove Newtonsoft.Json dependencies and implement System.Text.Json attributes.
- Ensured consistency in JSON serialization options across the codebase.

Author: reeshika-h