docs: update badge

- removes AppVeyor badge
- update Github Actions linux build badge with valid one

Author: WeihanLi

README.md

@@ -1,7 +1,6 @@
 ## RabbitMQ .NET Client
 
-[![AppVeyor Build status](https://ci.appveyor.com/api/projects/status/33srpo7owl1h3y4e?svg=true)](https://ci.appveyor.com/project/rabbitmq/rabbitmq-dotnet-client)
-![GitHub Actions (Linux)](https://github.com/rabbitmq/rabbitmq-dotnet-client/workflows/Build%20(Linux)/badge.svg)
+![GitHub Actions rabbitmq-dotnet-client](https://github.com/rabbitmq/rabbitmq-dotnet-client/actions/workflows/main.yaml/badge.svg)
 ![CodeQL](https://github.com/rabbitmq/rabbitmq-dotnet-client/workflows/CodeQL/badge.svg)
 
 This repository contains source code of the [RabbitMQ .NET client](https://www.rabbitmq.com/dotnet.html).