Update ci-testing.yml

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved notification setup for GitHub Actions workflow in YOLOv5.

📊 Key Changes

• Modified Slack notification setup in the CI workflow configuration.
• Changed webhook format to use incoming-webhook for sending alerts.
• Updated the payload section to utilize the webhook directly for error notifications.

🎯 Purpose & Impact

• Enhanced Notifications: The changes ensure better communication regarding CI job failures by simplifying and securing the Slack notification process.
• Improved Workflow Monitoring: Users and contributors can quickly receive updates and address issues, enhancing the efficiency of workflow management. 🚀

[UltralyticsAssistant]

👋 Hello @glenn-jocher, thank you for submitting a 🚀 PR to the ultralytics/yolov5 repository! To help us integrate your contribution smoothly, please ensure you review and follow the guidelines below:

• ✅ Define a Purpose: Ensure your PR description clearly explains its purpose, linking to any relevant issues if applicable. Commit messages should be clear and concise, adhering to our project's conventions.
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