Bump NUnit from 3.14.0 to 4.0.0 in /src

Bumps [NUnit](https://github.com/nunit/nunit) from 3.14.0 to 4.0.0.
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/master/CHANGES.md)
- [Commits](nunit/nunit@v3.14.0...v4.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

src/Tests/Tests.csproj

@@ -5,7 +5,7 @@
   <ItemGroup>
     <PackageReference Include="DocumentFormat.OpenXml" Version="3.0.0" />
     <PackageReference Include="MarkdownSnippets.MsBuild" Version="25.1.0" PrivateAssets="all" />
-    <PackageReference Include="NUnit" Version="3.14.0" />
+    <PackageReference Include="NUnit" Version="4.0.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
     <PackageReference Include="Verify.NUnit" Version="22.5.0" />