C# upgrade dotnet SDK (#6877)

* upgrade dotnet SDK

* upgrade dotnet SDK version in global.json

csharp/install_dotnet_sdk.ps1

@@ -19,3 +19,6 @@ $SDKVersion = $GlobalJson.sdk.version
 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

global.json

@@ -1,5 +1,5 @@
 {
   "sdk": {
-    "version": "2.1.504"
+    "version": "3.0.100"
   }
 }

kokoro/linux/dockerfile/test/csharp/Dockerfile

@@ -28,7 +28,8 @@ RUN apt-get update && apt-get install -y libunwind8 libicu57 && apt-get clean
 # Install dotnet SDK via install script
 RUN wget -q https://dot.net/v1/dotnet-install.sh && \
     chmod u+x dotnet-install.sh && \
-    ./dotnet-install.sh --version 2.1.504 && \
+    ./dotnet-install.sh --version 2.1.802 && \
+    ./dotnet-install.sh --version 3.0.100 && \
     ln -s /root/.dotnet/dotnet /usr/local/bin
 
 RUN wget -q www.nuget.org/NuGet.exe -O /usr/local/bin/nuget.exe