Add Immutable for net6

Author: twcclegg

csharp/PhoneNumbers/PhoneNumbers.csproj

@@ -42,6 +42,10 @@
     <PackageReference Include="System.Collections.Immutable" Version="1.7.1" />
   </ItemGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)' == 'net6.0' ">
+    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
+  </ItemGroup>
+
   <ItemGroup Condition="'$(TargetFramework)' == 'net7.0' ">
     <PackageReference Include="System.Collections.Immutable" Version="7.0.0" />
   </ItemGroup>