Rename to Open Workflow Spec - Prepare for v4

[ricardozanini]

Summary

• Rename from "Serverless Workflow" to "Open Workflow" across the entire codebase
• Update Go module path from github.com/serverlessworkflow/sdk-go/v3 to github.com/open-workflow-specification/sdk-go/v4
• Update error type URIs from serverlessworkflow.io to open-workflow-specification.org
• Update copyright headers, documentation, CI configs, and hack scripts to reflect the new branding

Notes

• No schema or API changes — this is a naming/branding update only
• Major version bump (v3 → v4) signals the module path change to consumers
• CNCF references (Code of Conduct, Slack) are intentionally kept