Fix package downgrade warning

Author: AArnott

Directory.Packages.props

@@ -31,7 +31,7 @@
     <PackageVersion Include="Microsoft.Win32.Registry" Version="5.0.0" />
     <PackageVersion Include="Microsoft.Windows.CsWin32" Version="0.3.162" />
     <PackageVersion Include="Nullable" Version="1.3.1" />
-    <PackageVersion Include="System.Collections.Immutable" Version="8.0.0" />
+    <PackageVersion Include="System.Collections.Immutable" Version="9.0.0" />
     <PackageVersion Include="System.Memory" Version="4.6.0" />
     <PackageVersion Include="System.Runtime.CompilerServices.Unsafe" Version="6.1.0" />
     <PackageVersion Include="System.Threading.Tasks.Extensions" Version="4.6.0" />
@@ -65,4 +65,4 @@
     <GlobalPackageReference Include="PolySharp" Version="1.15.0" />
     <GlobalPackageReference Include="StyleCop.Analyzers.Unstable" Version="1.2.0.556" />
   </ItemGroup>
-</Project>
+</Project>