Add changelog entry for switching on httpbin.org

Author: andrew-demb

CHANGELOG.md

@@ -6,6 +6,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [3.1.1] - 2024-xx-xx
+
+- Switched to `httpbin.org` for tests
+
 ## [3.1.0] - 2024-03-05
 
 - Removed builds for abandoned guzzle5 and guzzle6 adapters