Use PackageLicenseExpression, remove deprecated PackageLicenseUrl.

[bchavez]

Remove deprecated PackageLicenseUrl and use PackageLicenseExpression for main Bogus package. Fixes #499.

[bchavez]

There does not seem to be a way to maintain copyright holder names with PackageLicenseExpression only changes:

• [NuGet.org Bug]: SPDX templates for open source licenses do not include copyright information for the developer NuGet/NuGetGallery#10237

The <licenseUrl> points to license templates on NuGet.

• https://licenses.nuget.org/MIT%20AND%20BSD-3-Clause

I don't think it will be possible to omit the license URL if we want to maintain the copyright holder names.

[bchavez]

Build failure when trying to use both:

[bchavez]

Can't use PackageLicenseExpression and PackageLicenseFIle either: