Update action.yml

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancements to the GitHub Actions workflow for Ultralytics.

📊 Key Changes

• Branch Specification: Removed references to the 'master' branch, now exclusively using 'main'.
• Formatting Setup: Split the Markdown and YAML formatting tools; added prettier for YAML.
• Dependency Setup: Removed the Python setup step from the workflow.

🎯 Purpose & Impact

• Streamlined Branch Management: 🗂 Simplifies version control by focusing only on the 'main' branch.
• Improved Formatting: 🎨 Ensures YAML files are formatted with prettier, providing consistent code style.
• Efficiency Boost: 🚀 Reduces setup time and complexity by eliminating the Python setup step, potentially speeding up workflow runs.