Update .NET Core SDK to version 3.0.x.

azure-pipelines.yml

@@ -34,9 +34,17 @@ jobs:
   pool:
     vmImage: 'macOS-10.14'
   steps:
-  - task: DotNetCoreInstaller@0
+  - task: UseDotNet@2
+    displayName: 'Use .NET Core SDK 3.0.x'
     inputs:
-      version: '2.1.403'
+      packageType: sdk
+      version: 3.0.x
+
+  - task: UseDotNet@2
+    displayName: 'Use .NET Core Runtime 2.1.x'
+    inputs:
+      packageType: runtime
+      version: 2.1.x
 
   - task: CmdLine@2
     displayName: 'Install Mono 5.18'