Language server from VS Code requires .NET Core 3.1 SDK as well as .NET 6 SDK

[cgranade]

Describe the bug

When creating a new Q# project on a host with .NET 6 but not .NET Core 3.1 installed, the following error is observed:

To Reproduce

Steps to reproduce the behavior:

1. Start a new Windows Sandbox session
2. Install VS Code and .NET 6 SDK
3. Run VS Code and install the Quantum Development Kit extension from the marketplace.
4. From the command palette, select "Create a new Q# project..."

Expected behavior

The .NET 6 SDK should be a sufficient prerequisite to use the Quantum Development Kit extension.

System information

• Windows Sandbox: Version 10.0.22000.613
• VS Code: 1.66.2
• .NET SDK:

  > dotnet --list-sdks
  6.0.202 [C:\Program Files\dotnet\sdk]