Bump NUnit from 3.13.3 to 4.3.1 in /src

Bumps [NUnit](https://github.com/nunit/nunit) from 3.13.3 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@v3.13.3...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>

test/WalletFramework.MdocVc.Tests/WalletFramework.MdocVc.Tests.csproj

@@ -11,7 +11,7 @@
     <ItemGroup>
         <PackageReference Include="FluentAssertions" Version="6.12.0"/>
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0"/>
-        <PackageReference Include="NUnit" Version="3.13.3"/>
+        <PackageReference Include="NUnit" Version="4.3.1"/>
         <PackageReference Include="NUnit3TestAdapter" Version="4.2.1"/>
         <PackageReference Include="NUnit.Analyzers" Version="3.3.0"/>
         <PackageReference Include="xunit" Version="2.9.0" />