Add netstandard-specific package refs for Channels/Immutable

Added conditional PackageReferences for System.Threading.Channels and System.Collections.Immutable (min version 6.0.0) when targeting netstandard2.0 or netstandard2.1. This maintains compatibility for netstandard consumers without enforcing newer minimums, while allowing higher versions to resolve as needed.

Author: electricessence

Open.ChannelExtensions/Open.ChannelExtensions.csproj

@@ -49,6 +49,7 @@
 		</None>
 	</ItemGroup>
 
+	<!-- Minimum version only (not exact-pinned): keeps the floor low so netstandard consumers aren't forced to a newer minimum, while still allowing their own app's dependency graph to resolve a higher version if needed. -->
 	<ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0' or '$(TargetFramework)' == 'netstandard2.1'">
 		<PackageReference Include="System.Threading.Channels" Version="6.0.0" />
 		<PackageReference Include="System.Collections.Immutable" Version="6.0.0" />