feat: fix failing tests by upgrading System.Collections.Immutable (#297)

Author: wmundev

csharp/PhoneNumbers/PhoneNumbers.csproj

@@ -45,7 +45,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0' ">
-    <PackageReference Include="System.Collections.Immutable" Version="1.7.1" />
+    <PackageReference Include="System.Collections.Immutable" Version="8.0.0" />
     <PackageReference Include="System.IO.Compression" Version="4.3.0" />
   </ItemGroup>