eng: update Microsoft.NET.Test.Sdk from 17.12.0 to 18.4.0

Bumps the test SDK to the latest available version. No code changes;
all 414 tests pass.

Co-authored-by: Copilot <[email protected]>

Author: github-actions[bot]

tests/FSharp.Control.AsyncSeq.Tests/FSharp.Control.AsyncSeq.Tests.fsproj

@@ -15,7 +15,7 @@
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
     <ProjectReference Include="..\..\src\FSharp.Control.AsyncSeq\FSharp.Control.AsyncSeq.fsproj" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.4.0" />
     <PackageReference Include="NUnit" Version="3.9.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.6.0" />
     <PackageReference Include="System.Threading.Channels" Version="*" />