Remove requirements for VS 16.7

- use SignalR.Npm.FunctionalTests.npmproj to get non-stable version
  - not Microsoft.AspNetCore.DeveloperCertificates.XPlat.csproj (avoid C# and F# projects)
  - add `_GetPackageVersionInfo` target to all `*.npmproj` projects
    - make `_GetPackageVersionInfo` target work when `yarn` is not installed
- switch codesign-xplat.yml to use `dotnet msbuild`
  - above change also fixes Code-sign jobs but they're slightly faster using `dotnet msbuild`