Upgrade to .NET Core 3.1 (#1488)

[Jonarw]

Fixes #1488.
Also fixes #1483.

Checklist

• I have included examples or tests
• I have updated the change log
• I am listed in the CONTRIBUTORS file
• I have cleaned up the commit history (use rebase and squash)

Changes proposed in this pull request:

• Update all netcoreapp3.0 targets to netcoreapp3.1
• Change target order so net4xx targets are listed before netcoreapp3.x targets

Did I miss anything?

@oxyplot/admins

[Jonarw]

Azure Pipelines Build seems to fail for some reason... I don't really know what to do about this. @objorke could you take a look? Thanks!

[objorke]

The exceptions are thrown in a unit test in OxyPlot.Wpf.Tests
https://dev.azure.com/oxyplot/OxyPlot/_build/results?buildId=177&view=logs&j=12f1170f-54f2-53f3-20dd-22fc7dff55f9&t=7bfb7e0f-549d-5929-c603-6e8046613b9e

The appveyor build seems to work:
https://ci.appveyor.com/project/objorke/oxyplot/builds/31204899

[Jonarw]

Is it actually a failed unit test? Yes, there is an exception thrown during testing, but no test is reported as failed... On my machine all test pass (except ExportPngAndCompareWithBaseline, but I guess that one is tricky anyway and does actually pass on Azure).

I don't really know what to do about this. Can we just ignore this error?

[Jonarw]

I am going to go ahead and merge this, considering that the AppVeyor build works fine.