Add warning about JetBrains.Annotations

https://github.com/JetBrains/JetBrains.Annotations/issues/45

src/Directory.Build.props

@@ -45,7 +45,8 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="JetBrains.Annotations" Version="2024.3.0" PrivateAssets="all" />
+    <!-- Do not update beyond 2024.3.0, later versions do not use [Conditional] anymore -->
+    <PackageReference Include="JetBrains.Annotations" Version="[2024.3.0]" PrivateAssets="all" />
   </ItemGroup>
 
 </Project>