feat(firebase): upgrade default runtime to node 20

[luc122c]

🔗 Linked issue

N/A

❓ 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

Node 20 is the current LTS and the nodejs20 runtime is in GA on Google Cloud. It makes sense to update the fallback runtime and documentation to use the LTS.

📝 Checklist

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