Remove upper version constraint on System.Collections.Immutable (#75)

Author: simon-reynolds

Directory.Packages.props

@@ -5,7 +5,7 @@
     <PackageVersion Include="FSharp.Core" Version="[4.6.0,)" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="16.11.0" />
     <PackageVersion Include="Microsoft.TestPlatform.ObjectModel" Version="16.11.0" />
-    <PackageVersion Include="System.Collections.Immutable" Version="[1.5.0, 5.1)" />
+    <PackageVersion Include="System.Collections.Immutable" Version="[1.5.0,)" />
   </ItemGroup>
 
 </Project>