CI badge update [skip ci]

Author: nblumhardt

README.md

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-# Serilog.Extensions.Logging [![Build status](https://ci.appveyor.com/api/projects/status/865nohxfiq1rnby0/branch/master?svg=true)](https://ci.appveyor.com/project/serilog/serilog-framework-logging/history) [![NuGet Version](http://img.shields.io/nuget/v/Serilog.Extensions.Logging.svg?style=flat)](https://www.nuget.org/packages/Serilog.Extensions.Logging/)
+# Serilog.Extensions.Logging [![Build status](https://github.com/serilog/serilog-extensions-logging/actions/workflows/ci.yml/badge.svg?branch=dev)](https://github.com/serilog/serilog-extensions-logging/actions) [![NuGet Version](http://img.shields.io/nuget/v/Serilog.Extensions.Logging.svg?style=flat)](https://www.nuget.org/packages/Serilog.Extensions.Logging/)
 
 A Serilog provider for [Microsoft.Extensions.Logging](https://www.nuget.org/packages/Microsoft.Extensions.Logging), the logging subsystem used by ASP.NET Core.