nuget <licenseUrl> is deprecated

[blumendorf]

We saw a couple of urls like this for our .net projects: https://aka.ms/deprecateLicenseUrl

This is related to the <licenseUrl> tag being deprecated and microsoft using the <license> tag instead: NuGet/Announcements#32

See https://www.nuget.org/packages/HotChocolate for an example.

The new license tag looks like this
<license type="expression">MIT</license> or <license type="file">LICENSE.txt</license>

So, I believe that the nuspec_license_urls method in the nuget package manager needs to be changed.

Unfortunately, I have never used ruby, so I am having trouble making a PR.
I also had trouble fully understanding the matcher logic.
Here is some rough idea, but it probably needs a slightly more sophisticated approach.

    def self.nuspec_license_urls(filepath)
      content = File.read(filepath)
      dir = File.dirname(filepath)
      xml = REXML::Document.new(content)
      licenses = REXML::XPath.match(xml, '//metadata//license')

      # if no licenses are found, fallback to licenseUrl
      if (licenses.length == 0)
        return REXML::XPath.match(xml, '//metadata//licenseUrl')
                  .map(&:get_text)
                  .map(&:to_s)
      end

      licenses.map do |license|
        type = license.attributes['type']

        if type == 'expression'
          # return the expressions directly
          license.get_text.to_s

        elsif type == 'file'
          # read the file and do something useful with it
          license_file_name = license.get_text.to_s.strip
          license_file = File.join(dir, license_file_name)
          if File.exist?(license_file)
            file_content = File.read(license_file).strip
            # not sure what to do here, since I could not get this to work with the matcher
          else
            # log an error and return something empty
          end
        end
      end
    end

I am not capable of producing a PR for this, but I am happy to team up with anyone willing to tackle this.

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