update documentation to reference dotnet 8

Author: kengibous

samples/bot-receive-channel-messages-withRSC/csharp/ReceiveMessagesWithRSC/README.md

@@ -11,7 +11,7 @@ This feature shown in this sample is currently available in Public Developer Pre
 
 ## Prerequisites
 
-- [.NET Core SDK](https://dotnet.microsoft.com/download) version 6.0
+- [.NET Core SDK](https://dotnet.microsoft.com/download) version 8.0
 
   ```bash
   # determine dotnet version