Bump NUnit from 3.13.3 to 4.0.0 in /Backend.Tests

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

Diff for: Backend.Tests/Backend.Tests.csproj

@@ -13,7 +13,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.2" />
-    <PackageReference Include="NUnit" Version="3.13.3" />
+    <PackageReference Include="NUnit" Version="4.0.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
     <PackageReference Include="coverlet.collector" Version="6.0.0"/>
     <PackageReference Include="coverlet.msbuild" Version="6.0.0"/>