nuget: bump NUnit from 3.13.3 to 4.1.0 in the nunit group

Bumps the nunit group with 1 update: [NUnit](https://github.com/nunit/nunit).


Updates `NUnit` from 3.13.3 to 4.1.0
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/master/CHANGES.md)
- [Commits](https://github.com/nunit/nunit/compare/v3.13.3...4.1.0)

---
updated-dependencies:
- dependency-name: NUnit
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: nunit
...

Signed-off-by: dependabot[bot] <support@github.com>

Directory.Packages.props

@@ -25,7 +25,7 @@
     <PackageVersion Include="Microsoft.IO.RecyclableMemoryStream" Version="3.0.1" />
     <PackageVersion Include="MsgPack.Cli" Version="1.0.1" />
     <PackageVersion Include="NetCoreServer" Version="8.0.7" />
-    <PackageVersion Include="NUnit" Version="3.13.3" />
+    <PackageVersion Include="NUnit" Version="4.1.0" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.1.0" />
     <PackageVersion Include="OpenTK.Core" Version="4.8.2" />
     <PackageVersion Include="OpenTK.Graphics" Version="4.8.2" />