Put commandline tool back to 3.1 now that pipeline has new Nuget

Author: darrelmiller

src/Microsoft.OpenApi.Tool/Microsoft.OpenApi.Tool.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp2.2</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <PackAsTool>true</PackAsTool>
     <ToolCommandName>openapi</ToolCommandName>
     <PackageOutputPath>./../../artifacts</PackageOutputPath>