Revert "Revert "C# upgrade dotnet SDK (#6877)" (#6888)" (#6920)

This reverts commit f06800524f0fe0d2e71a2d3696ce7a5278836445.
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
diff --git a/global.json b/global.json
index 0c7cc9f..1c02f07 100644
--- a/global.json
+++ b/global.json
@@ -1,5 +1,5 @@
 {
   "sdk": {
-    "version": "2.1.504"
+    "version": "3.0.100"
   }
 }
diff --git a/kokoro/linux/dockerfile/test/csharp/Dockerfile b/kokoro/linux/dockerfile/test/csharp/Dockerfile
index 2073057..3d54436 100644
--- a/kokoro/linux/dockerfile/test/csharp/Dockerfile
+++ b/kokoro/linux/dockerfile/test/csharp/Dockerfile
@@ -28,7 +28,8 @@
 # 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