Bump NUnit from 3.14.0 to 4.3.1 in /lang/csharp

Bumps [NUnit](https://github.com/nunit/nunit) from 3.14.0 to 4.3.1.
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/main/CHANGES.md)
- [Commits](nunit/nunit@v3.14.0...4.3.1)

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

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

lang/csharp/versions.props

@@ -68,7 +68,7 @@
     <MicrosoftCodeAnalysisCSharpCodeStyleVersion>4.7.0</MicrosoftCodeAnalysisCSharpCodeStyleVersion>
     <MicrosoftCodeAnalysisNetAnalyzersVersion>8.0.0</MicrosoftCodeAnalysisNetAnalyzersVersion>
     <MicrosoftNETTestSdkVersion>17.8.0</MicrosoftNETTestSdkVersion>
-    <NUnitVersion>3.14.0</NUnitVersion>
+    <NUnitVersion>4.3.1</NUnitVersion>
     <NUnitConsoleRunnerVersion>3.16.3</NUnitConsoleRunnerVersion>
     <NUnit3TestAdapterVersion>4.5.0</NUnit3TestAdapterVersion>
     <StyleCopAnalyzersVersion>1.1.118</StyleCopAnalyzersVersion>