Update dependency NUnit3TestAdapter to v6

Author: renovate[bot]

AngleSharp.ContentExtraction.IntegrationTests/AngleSharp.ContentExtraction.IntegrationTests.csproj

@@ -8,7 +8,7 @@
 
   <ItemGroup>
     <PackageReference Include="nunit" Version="4.4.0" />
-    <PackageReference Include="NUnit3TestAdapter" Version="5.2.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="6.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
   </ItemGroup>