Updating SDK for appveyor build

Author: batzen

appveyor.yml

@@ -13,7 +13,7 @@ pull_requests:
 
 install:
   - ps: Invoke-WebRequest 'https://dot.net/v1/dotnet-install.ps1' -OutFile 'dotnet-install.ps1'
-  - ps: ./dotnet-install.ps1 -Version 6.0.300 -InstallDir "C:\Program Files\dotnet"
+  - ps: ./dotnet-install.ps1 -Version 6.0.400 -InstallDir "C:\Program Files\dotnet"
 
 build_script:
   - cmd: dotnet --info