Update links to MessagePack-CSharp (dotnet#16640)

One of the anchor links was out of date. But really both links needed updating because they point people at the 2.1 documentation when for SignalR only the 1.x version is relevant. So I've updated the links to explicitly point at the v1.8 branch of the docs which fixes the broken link and points people at the docs relevant to the version they're using.

Author: AArnott

aspnetcore/signalr/messagepackhubprotocol.md

@@ -166,7 +166,7 @@ For more information on this limitation, see GitHub issue [aspnet/SignalR#2228](
 
 ### MessagePack support in "ahead-of-time" compilation environment
 
-The [MessagePack-CSharp](https://github.com/neuecc/MessagePack-CSharp) library used by the .NET client and server uses code generation to optimize serialization. As a result, it isn't supported by default on environments that use "ahead-of-time" compilation (such as Xamarin iOS or Unity). It's possible to use MessagePack in these environments by "pre-generating" the serializer/deserializer code. For more information, see [the MessagePack-CSharp documentation](https://github.com/neuecc/MessagePack-CSharp#pre-code-generationunityxamarin-supports). Once you have pre-generated the serializers, you can register them using the configuration delegate passed to `AddMessagePackProtocol`:
+The [MessagePack-CSharp](https://github.com/neuecc/MessagePack-CSharp/tree/v1.8) library used by the .NET client and server uses code generation to optimize serialization. As a result, it isn't supported by default on environments that use "ahead-of-time" compilation (such as Xamarin iOS or Unity). It's possible to use MessagePack in these environments by "pre-generating" the serializer/deserializer code. For more information, see [the MessagePack-CSharp documentation](https://github.com/neuecc/MessagePack-CSharp/tree/v1.8#pre-code-generationunityxamarin-supports). Once you have pre-generated the serializers, you can register them using the configuration delegate passed to `AddMessagePackProtocol`:
 
 ```csharp
 services.AddSignalR()