Fix licenseUrl element in nuspec, will be deprecated

[mikkelbu]

Warning when generating nuget packages. See e.g. https://ci.appveyor.com/project/CharliePoole/nunit/builds/22954283

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PackageFramework
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Attempting to build package from 'nunit.temp.nuspec'.
Successfully created package 'C:\projects\nunit\package\NUnit.3.12.0-ci-06156-pr-3184.nupkg'.
WARNING: NU5125: The 'licenseUrl' element will be deprecated. Consider using the 'license' element instead.
Attempting to build package from 'nunitlite.temp.nuspec'.
Successfully created package 'C:\projects\nunit\package\NUnitLite.3.12.0-ci-06156-pr-3184.nupkg'.
WARNING: NU5125: The 'licenseUrl' element will be deprecated. Consider using the 'license' element instead.

[parrainc]

If no one have done something yet, I can take some few mins to fix this later today.

@mikkelbu question, according to the docs here it says that we should use <license type="expression">..</license> with a SPDX license id (in nunit's case would be MIT). So, just wondering if it would be ok changing the LICENSE.txt file path with this identifier in the nuspec file? Otherwise I'll have to add type="file"

I'll be waiting for response to create the PR.

[mikkelbu]

We would love a PR @parrainc. Regarding of whether we should go with type="expression" or type="file" I'm leaning towards file, but that is mostly because I'm no expert in licensens. @nunit/framework-team do any of you have any opinion about this? (will linking to a general MIT licens from nuget be fine or should we rather link to our file that mentions Charlie and Rob?)

[parrainc]

@mikkelbu me as well, I'm leaning for the second one as well since in the .nuspec files the LICENSE.txt is being deployed as part of the nuget package, and nuget recommends that if we're going to use type file for licenses.

PR is created with type="file", if the team has any suggestion/opinion on going with file or expression, I can update the same.

[mikkelbu]

This was fixed in #3192.