[nuget] use $(PackageLicenseExpression), not $(PackageLicenseUrl) (#35)

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][0] 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][1] the [`<licenseUrl/>`][2]
element, and MSBuild deprecated the [`$(PackageLicenseUrl)`][3]
MSBuild property.  The NuGet [`<license/>`][4] element or
[`$(PackageLicenseExpression)` MSBuild property][5] should be used.

Replace `$(PackageLicenseUrl)` with `$(PackageLicenseExpression)`,
with a value of `MIT AND BSD-3-Clause`.  The value needs to be an
[SPDX identifier][6], 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.

Update `$(_OpenTKNugetVersion)` to 1.0.2.

[0]: https://github.com/clearlydefined/curated-data/
[1]: NuGet/Home#7509
[2]: https://docs.microsoft.com/en-us/nuget/reference/nuspec#licenseurl
[3]: https://docs.microsoft.com/en-us/nuget/reference/msbuild-targets#pack-target
[4]: https://docs.microsoft.com/en-us/nuget/reference/nuspec#license
[5]: https://docs.microsoft.com/en-us/nuget/reference/msbuild-targets#packing-a-license-expression-or-a-license-file
[6]: https://spdx.org/licenses/

Try to fix DllNotFoundException under .net 6 (#32)

* Try to fix DllNotFoundException under .net 6

Fixes #31

Merge pull request #30 from dellis1972/proguard

Add Proguard config support

Create Xamarin.Legacy.OpenTK Nuget Package (#28)

Create a new Xamarin.Legacy.OpenTK NuGet package.

This will replace `OpenTK-1.0.dll`, currently shipped with
Xamarin.Android.