Replace Slack with Community Forum in issues

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update to issue reporting channels to direct users to the Ultralytics Community Forum.

📊 Key Changes

• Removed Slack contact link for issue reporting.
• Added a new contact link for the Ultralytics Community Forum.

🎯 Purpose & Impact

• Purpose: To streamline user support by guiding them to the newly established community forum rather than using Slack.
• Impact: Users reporting issues or seeking help will now be directed to a dedicated forum which could offer better-structured support and community engagement. 🧑‍🤝‍🧑