Bump nunit from 3.13.3 to 4.0.1

Bumps [nunit](https://github.com/nunit/nunit) from 3.13.3 to 4.0.1.
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/master/CHANGES.md)
- [Commits](nunit/nunit@v3.13.3...v4.0.1)

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

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

tests/Webserver.API.UnitTests/Webserver.Api.UnitTests.csproj

@@ -21,7 +21,7 @@
 			<IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
 		</PackageReference>
 		<PackageReference Include="MimeMapping" Version="1.0.1.37" />
-		<PackageReference Include="nunit" Version="3.13.3" />
+		<PackageReference Include="nunit" Version="4.0.1" />
 		<PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
 		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
 		<PackageReference Include="RichardSzalay.MockHttp" Version="6.0.0" />