Replace deprecated licenseUrl field with license one in .nuspec file

[StrikerRUS]

The 'licenseUrl' element will be deprecated. Consider using the 'license' element instead.
https://docs.microsoft.com/en-us/nuget/reference/errors-and-warnings/nu5125
https://docs.microsoft.com/en-us/nuget/reference/nuspec#license
NuGet/Announcements#32
#3872 (comment)

[jameslamb]

👍

[github-actions]

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