Replace canned action to push to nuget with dotnet commands

[JYCabello]

Example:

- name: Package
  run: dotnet pack -c Release -o . GrpcBrowser/GrpcBrowser.csproj
- name: Publish
  run: dotnet nuget push *.nupkg -k ${{ secrets.NUGET_KEY }} -s https://api.nuget.org/v3/index.json