onnxruntime

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.