onnxruntime

c352e9b1 - Rework/cleanup the C# build infrastructure for nuget packages. (#18127)

Rework/cleanup the C# build infrastructure for nuget packages. (#18127) ### Description Update the C# nuget build infrastructure to make building a test nuget package more user friendly and to simplify - Remove usage of dotnet and msbuild in CIs - was temporary requirement until .net 6 MAUI was added to the released Visual Studio - remove SelectedTargets property and its usage - Add property for excluding mobile targets - generally we exclude based on the nuget package name - can now specify `/p:IncludeMobileTargets=false` on the command line to force exclusion - support building test package using build.py `--build_nuget` better - limit inclusion of xamarin targets as building with them requires a lot more infrastructure - use msbuild directly if xamarin targets are included. use dotnet otherwise. - remove quoting of property values as it doesn't appear to be necessary and breaks when msbuild is being used - add infrastructure to be able to pack the nuget package on linux with `dotnet pack` - `nuget pack` is not user friendly as-per comments in changes - requires stub csproj to provide the nuspec path - Remove netstandard1.0 targets from nuspec - we removed support from the actual bindings previously - Remove usage of nuget-staging directory when creating nuget package on linux - the nuspec file element has a fully qualified path for a source file so there is no obvious benefit to copying to a staging directory prior to packing ### Motivation and Context Address issues with 1P users trying to create test nuget packages locally. Long overdue cleanup of CI complexity.