Update dependency NUnit to v4

Author: renovate[bot]

ScipDotnet.Tests/ScipDotnet.Tests.csproj

@@ -10,7 +10,7 @@
     <ItemGroup>
         <PackageReference Include="DiffPlex" Version="1.7.1" />
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.1.0" />
-        <PackageReference Include="NUnit" Version="3.13.3" />
+        <PackageReference Include="NUnit" Version="4.2.0" />
         <PackageReference Include="NUnit3TestAdapter" Version="4.2.1" />
         <PackageReference Include="NUnit.Analyzers" Version="3.3.0" />
         <PackageReference Include="coverlet.collector" Version="3.1.2" />