Bump actions/checkout from v4 to v6 in /.github/workflows

[UltralyticsAssistant]

Bump actions/checkout from v4 to v6 in /.github/workflows

This PR updates GitHub Actions references in this repository.

Examples:

• Branch: actions/checkout@main stays on @main
• Major tag: actions/checkout@v4 updates to @v5
• Specific tag: astral-sh/setup-uv@v0.5.0 updates to @v0.9.4
• SHA pinned: actions/checkout@<sha> # v4.1.0 updates to the latest release SHA and tag comment

_{Made with ❤️ by Ultralytics Actions}

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

🔄 This PR updates the GitHub Actions checkout step in CI from actions/checkout@v4 to actions/checkout@v6 to keep the repository’s automation current.

📊 Key Changes

• 🛠️ Updated .github/workflows/ci.yml
• ⬆️ Changed the CI checkout action from actions/checkout@v4 to actions/checkout@v6
• 🤖 No changes to JSON2YOLO conversion logic, features, or user-facing functionality

🎯 Purpose & Impact

• ✅ Keeps the CI workflow aligned with newer GitHub Actions versions and best practices
• 🔒 May improve security, compatibility, and long-term maintenance of the project’s automation
• 🚀 Helps ensure continued reliability of automated testing and validation
• 📦 Has no direct impact on how end users run or use JSON2YOLO, since this is a workflow-only update