Update greetings.yml

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improvements to GitHub Greetings Workflow for new PRs and Issues.

📊 Key Changes

• Modified the triggers for the GitHub greeting actions to activate only when PRs or Issues are opened.
• The greetings will no longer be sent on other types of pull request and issue events.

🎯 Purpose & Impact

• Efficiency: Reduces unnecessary workflow runs, saving actions minutes and reducing noise.
• Focus: Maintains a friendly welcome message for new contributors while streamlining the workflow experience.
• User Experience: Contributors will receive a greeting only when they first open PRs or Issues, making the interaction more relevant and timely.🤖✨