Update dependency Microsoft.NET.Test.Sdk to v18

Author: renovate[bot]

src/Cake.Markdownlint.Tests/Cake.Markdownlint.Tests.csproj

@@ -15,7 +15,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
     <PackageReference Include="Cake.Core" Version="4.0.0" />
     <PackageReference Include="Cake.Testing" Version="4.0.0" />
     <PackageReference Include="Shouldly" Version="4.3.0" />