Use MIT SPDX license identifier in Nuget metadata

[tsondergaard]

licenseUrl is deprecated in the nuget spec. Using an SPDX identifier for the license is the recommended approach.

https://learn.microsoft.com/en-us/nuget/reference/nuspec#licenseurl
https://learn.microsoft.com/en-us/nuget/reference/msbuild-targets#packing-a-license-expression-or-a-license-file

This is of practical importance for downstream projects that use automatic tools to inspect and document licenses. This is the "licenses" element that dotnet-CycloneDX currently generates for NReco.Logging:

"licenses": [
  {
    "license": {
      "name": "Unknown - See URL",
      "url": "https://raw.githubusercontent.com/nreco/logging/master/LICENSE"
    }
  }
]

With this commit it becomes simply:

"licenses": [
  {
    "license": {
      "id": "MIT"
    }
  }
]