Document removal of Quack

CHANGELOG.md

@@ -56,6 +56,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Fixed lowercase HTTP HEAD method in the Media Proxy Preview code
 
 ### Removed
+- Quack, the logging backend that pushes to Slack channels
 
 ## 2.4.4 - 2022-08-19