There is no step "Select an OS" when creating a .NET 9 (STS) (Preview) advanced web app

[v-meilei]

OS: All App Service Build Version: 20240827.1 Regression: Not a regression

Repro Steps:

1. F1 -> Azure App Service: Create New Web App... (Advanced) -> Select a subscription -> Enter a name -> Select a resource -> Select the ".NET 9 (STS) (Preview)" runtime stack.
2. Check whether the next step is “Select an OS".

Expect: The next step is "Select an OS".

Actual: The next step is "Select a location for new resources.".

More Info:

1. This issue doesn't reproduce when creating .NET 8 (LTS)/.NET 7 (STS)/.NET 6 (LTS) web app.
2. The workaround is to enable "App Service: Show Hidden Stacks" in Settings for VS Code/VS Code-Insiders.