C# upgrade dotnet SDK (#6877)

* upgrade dotnet SDK

* upgrade dotnet SDK version in global.json

csharp/install_dotnet_sdk.ps1

diff --git a/csharp/install_dotnet_sdk.ps1 b/csharp/install_dotnet_sdk.ps1
index b4132ab..9e300ed 100644
--- a/csharp/install_dotnet_sdk.ps1
+++ b/csharp/install_dotnet_sdk.ps1
@@ -19,3 +19,6 @@
 Write-Host "Downloading install script: $InstallScriptUrl => $InstallScriptPath"
 Invoke-WebRequest -Uri $InstallScriptUrl -OutFile $InstallScriptPath
 &$InstallScriptPath -Version $SDKVersion
+
+# Also install dotnet SDK LTS which is required to run some of the tests
+&$InstallScriptPath -Version 2.1.802