onnxruntime

81831201 - Change C# tests to use C# 5.0 (#10686)

Change C# tests to use C# 5.0 (#10686) .NET Core 2.1 has reached end of support on August 21, 2021. Use C# 5.0 instead. Our CI build machines do no have C# 6.0 yet. Later I will do it.