[nuget] use $(PackageLicenseExpression), not $(PackageLicenseUrl)

[jonpryor]

Context: https://docs.microsoft.com/en-us/nuget/reference/nuspec#licenseurl Context: https://docs.microsoft.com/en-us/nuget/reference/nuspec#license Context: https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/nuget#important-nuget-package-metadata Context: https://docs.clearlydefined.io/curation-guidelines

Certain internal Microsoft tools check NuGet license information by using ClearlyDefined curated data to determine verified license information for packages which don't "clearly define" their license.

Unfortunately, a license URL is not considered "clearly defined", likely because it isn't really machine readable. (Just because the URL contains "MIT" doesn't mean it's actually MIT.)

As part of this effort, NuGet deprecated the <licenseUrl/> element, and MSBuild deprecated the $(PackageLicenseUrl) MSBuild property. The NuGet <license/> element or $(PackageLicenseExpression) MSBuild property should be used.

Replace $(PackageLicenseUrl) with $(PackageLicenseExpression), with a value of MIT AND BSD-3-Clause. The value needs to be an SPDX identifier, and as OpenTK contains both MIT and 3-clause BSD license text within Documentation/Licennse.txt, using the AND expression feels appropriate.

This change should satisfy ClearlyDefined.

[dellis1972]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[jonpryor]

@dellis1972: version number bumped.