upgrade dotnet SDK in the CI to 3.1
diff --git a/csharp/install_dotnet_sdk.ps1 b/csharp/install_dotnet_sdk.ps1
index 9e300ed..8bc967c 100755
--- a/csharp/install_dotnet_sdk.ps1
+++ b/csharp/install_dotnet_sdk.ps1
@@ -1,5 +1,5 @@
#!/usr/bin/env powershell
-# Install dotnet SDK based on the SDK version from global.json
+# Install dotnet SDK using the official dotnet-install.ps1 script
Set-StrictMode -Version 2
$ErrorActionPreference = 'Stop'
@@ -9,16 +9,12 @@
$InstallScriptUrl = 'https://dot.net/v1/dotnet-install.ps1'
$InstallScriptPath = Join-Path "$env:TEMP" 'dotnet-install.ps1'
-$GlobalJsonPath = Join-Path $PSScriptRoot '..' | Join-Path -ChildPath 'global.json'
-
-# Resolve SDK version from global.json file
-$GlobalJson = Get-Content -Raw $GlobalJsonPath | ConvertFrom-Json
-$SDKVersion = $GlobalJson.sdk.version
# Download install script
Write-Host "Downloading install script: $InstallScriptUrl => $InstallScriptPath"
Invoke-WebRequest -Uri $InstallScriptUrl -OutFile $InstallScriptPath
-&$InstallScriptPath -Version $SDKVersion
-# Also install dotnet SDK LTS which is required to run some of the tests
+# The SDK versions to install should be kept in sync with versions
+# installed by kokoro/linux/dockerfile/test/csharp/Dockerfile
&$InstallScriptPath -Version 2.1.802
+&$InstallScriptPath -Version 3.1.301
