* Update to .NET Core 3.1
This update was faily trivial change of project properties with only one caveat
```
error CS0104: 'Range' is an ambiguous reference between 'Microsoft.ML.Probabilistic.Models.Range' and 'System.Range'
```
which require sprinkling everywhere following line of code
```
using Range = Microsoft.ML.Probabilistic.Models.Range;
```
* Update build definition to use more modern images
- Windows 2019 for .NET Core 3.1
- Mac OS 10.14 (Specifically https://docs.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops)
Co-authored-by: Tom Minka <8955276+tminka@users.noreply.github.com>
Andrii Kurdiumov