Update dependency Microsoft.NET.Test.Sdk to v18

Author: renovate[bot]

SIL.NuGetCleaner.Tests/SIL.NuGetCleaner.Tests.csproj

@@ -12,7 +12,7 @@
     <PackageReference Include="NuGet.Versioning" Version="6.14.0" />
     <PackageReference Include="nunit" Version="4.4.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.6.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
     <PackageReference Include="RichardSzalay.MockHttp" Version="7.0.0" />
   </ItemGroup>