Upgrade to netcoreapp3.1

[Jonarw]

.NET Core 3.0 reaches end of Life today. We should upgrade all relevant packages to .NET Core 3.1, which is a LTS release. Upgrading should be straightforward I think. This affects:

• OxyPlot.WPF
• OxyPlot.WindowsForms

[Jonarw]

Could combine this with #1483.