Update dependency Microsoft.NET.Test.Sdk to 18.6.0 (#247)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

tests/Markdown.ColorCode.UnitTests/Markdown.ColorCode.UnitTests.csproj

@@ -23,7 +23,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
     <PackageReference Include="HtmlAgilityPack" Version="1.12.4" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.5.1" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.6.0" />
     <PackageReference Include="NSubstitute" Version="5.3.0" />
     <PackageReference Include="NSubstitute.Analyzers.CSharp" Version="1.0.17">
       <PrivateAssets>all</PrivateAssets>