Adding netcoreapp3.1 as a target framework (#509)

* Adding netcoreapp3.0 and netcoreapp3.1 as a target framework

* Expanding NETSTANDARD check to include ASP.NET Core targets

* End of life netcoreapp3.0 changes and using NETCOREAPP3_1 instead of USE_ASPNETCORE where possible

src/WireMock.Net/Matchers/CSharpCodeMatcher.cs

@@ -151,7 +151,7 @@ namespace WireMock.Matchers
                 throw new WireMockException("CSharpCodeMatcher: Problem calling method 'IsMatch' in WireMock.CodeHelper", ex);
             }
 
-#elif (NETSTANDARD2_0 || NETSTANDARD2_1)
+#elif (NETSTANDARD2_0 || NETSTANDARD2_1 || NETCOREAPP3_1)
             dynamic script;
             try
             {