Update .NET SDKs to LTS versions
diff --git a/csharp/install_dotnet_sdk.ps1 b/csharp/install_dotnet_sdk.ps1
index c78655c..f9dc0d9 100755
--- a/csharp/install_dotnet_sdk.ps1
+++ b/csharp/install_dotnet_sdk.ps1
@@ -16,5 +16,5 @@
# The SDK versions to install should be kept in sync with versions
# installed by kokoro/linux/dockerfile/test/csharp/Dockerfile
-&$InstallScriptPath -Version 2.1.802
-&$InstallScriptPath -Version 5.0.102
+&$InstallScriptPath -Version 3.1.415
+&$InstallScriptPath -Version 6.0.100