Related command
az functionapp create --functions-version 4 (without specifying --runtime)

Is your feature request related to a problem? Please describe.
If no runtime is specified, the CLI will choose "dotnet" as per:

This will target .NET 6 on the in-process model, when for a .NET customer it would be better to target .NET 8 using the "dotnet-isolated" runtime.

This defaulting also occurs silently. The user does not know that they have done this. And if the user DID want to specify something else, they then have to know to go set the FUNCTIONS_WORKER_RUNTIME setting.

Describe the solution you'd like
An error or at least a strong warning in this case. --runtime should basically be required. It's not appropriate to be biasing to one runtime, and we want to ensure FUNCTIONS_WORKER_RUNTIME is always set, and set in accordance with user intent.

Describe alternatives you've considered
Help text shows using this too aggressively. We could change the text to show other forms. Probably both are required.

Additional context
Related to #28224