Readd basetargets in Microsoft.ML.OnnxRuntime.csproj (#18789)

### Description
<!-- Describe your changes. -->



### Motivation and Context
Now, the nightly Microsoft.ML.Onnxruntime.Managed Nuget Packag couldn't
be added in dotnet console program in VS2022 with target framework .NET
6.0.
I just restore it to previous setting to make it work.

csharp/src/Microsoft.ML.OnnxRuntime/Microsoft.ML.OnnxRuntime.csproj

@@ -6,7 +6,7 @@
 
   <PropertyGroup>
     <IncludeMobileTargets>true</IncludeMobileTargets>
-    <BaseTargets>netstandard2.0</BaseTargets>
+    <BaseTargets>netstandard2.0;netcoreapp3.1;net6.0</BaseTargets>
     <MobileTargets></MobileTargets>
   </PropertyGroup>