Bump NUnit from 4.2.0 to 4.2.1 in /src

Bumps [NUnit](https://github.com/nunit/nunit) from 4.2.0 to 4.2.1.
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/main/CHANGES.md)
- [Commits](nunit/nunit@4.2.0...4.2.1)

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

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

Author: dependabot[bot]

src/Orc.Plot.Tests/Orc.Plot.Tests.csproj

@@ -21,7 +21,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.0" />
     <PackageReference Include="ModuleInit.Fody" Version="2.1.1" PrivateAssets="all" />
     <PackageReference Include="Moq" Version="4.20.70" />
-    <PackageReference Include="NUnit" Version="4.2.0" PrivateAssets="all" />
+    <PackageReference Include="NUnit" Version="4.2.1" PrivateAssets="all" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.6.0">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>