Update SG to latest roslyn

Ix.NET/Source/System.Linq.Async.SourceGenerator/System.Linq.Async.SourceGenerator.csproj

@@ -5,7 +5,7 @@
     <IsPackable>false</IsPackable>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.9.0" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.0.1" PrivateAssets="all" />
     <PackageReference Include="IsExternalInit" Version="1.0.2" PrivateAssets="all" />
   </ItemGroup>
 </Project>