Bump Microsoft.NET.Test.Sdk from 17.13.0 to 18.6.0

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-version: 18.6.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

tests/NLog.Web.AspNetCore.Tests/NLog.Web.AspNetCore.Tests.csproj

@@ -18,7 +18,7 @@
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\src\NLog.Web.AspNetCore\NLog.Web.AspNetCore.csproj" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.6.0" />
     <PackageReference Include="NSubstitute" Version="5.3.0" />
     <PackageReference Include="xunit" Version="2.9.3" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">