Upgrade to .net standard 2.1

Author: li-haobin

O2DESNet/O2DESNet.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <Version Condition=" '$(BUILD_BUILDNUMBER)' == '' ">1.0.0.0</Version>
     <Version Condition=" '$(BUILD_BUILDNUMBER)' != '' ">$(BUILD_BUILDNUMBER)</Version>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFramework>netstandard2.1</TargetFramework>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
     <Authors>Li Haobin</Authors>
     <Description>A framework for Object-Oriented Discrete Event Simulation</Description>