Bump NSubstitute from 5.3.0 to 6.0.0

---
updated-dependencies:
- dependency-name: NSubstitute
  dependency-version: 6.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

test/LibYear.Core.Tests/LibYear.Core.Tests.csproj

@@ -7,7 +7,7 @@
 	<ItemGroup>
 		<PackageReference Include="coverlet.collector" Version="10.0.1" />
 		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.7.0" />
-		<PackageReference Include="NSubstitute" Version="5.3.0" />
+		<PackageReference Include="NSubstitute" Version="6.0.0" />
 		<PackageReference Include="xunit" Version="2.9.3" />
 		<PackageReference Include="xunit.runner.visualstudio" Version="3.1.5" />
 		<ProjectReference Include="../../src/LibYear.Core/LibYear.Core.csproj" />