chore: remove experimental comment for event.waitUntil

[danielroe]

🔗 Linked issue

#1420

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

waitUntil is no longer experimental and we can unflag this.

Could follow up with adding some more info in the docs?

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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