build: use latest .NET Core SDK 3.1.201 to build

Use the latest .NET Core 3.1.x SDK to build since we are using the
new WindowsDesktop project SDK. We are not targeting .NET Core 3.1 yet,
but need the newer toolchain.

Author: mjcheetham

.azure-pipelines/templates/osx/compile.yml

@@ -1,9 +1,9 @@
 steps:
 - task: DotNetCoreInstaller@0
-  displayName: Install .NET Core SDK 2.2.100
+  displayName: Install .NET Core SDK 3.1.201
   inputs:
     packageType: sdk
-    version: '2.2.100'
+    version: '3.1.201'
 
 - task: DotNetCoreCLI@2
   displayName: Compile common code and macOS Helpers

.azure-pipelines/templates/osx/pack.signed/step3-pack.yml

@@ -7,10 +7,10 @@ steps:
       downloadPath: '$(Build.StagingDirectory)/payload'
 
   - task: DotNetCoreInstaller@0
-    displayName: Install .NET Core SDK 2.2.100
+    displayName: Install .NET Core SDK 3.1.201
     inputs:
       packageType: sdk
-      version: '2.2.100'
+      version: '3.1.201'
 
   - script: dotnet tool install --global nbgv
     displayName: Install Nerdbank.GitVersioning tool

.azure-pipelines/templates/windows/compile.yml

@@ -6,10 +6,10 @@ steps:
     signType: '$(SignType)'
 
 - task: DotNetCoreInstaller@0
-  displayName: Install .NET Core SDK 2.2.100
+  displayName: Install .NET Core SDK 3.1.201
   inputs:
     packageType: sdk
-    version: '2.2.100'
+    version: '3.1.201'
 
 - task: NuGetToolInstaller@0
   displayName: Install NuGet tool >=4.3.0