add changelog entry

Author: graingert-coef

CHANGELOG.md

@@ -7,6 +7,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ## Changed
 
 - [#2734](https://github.com/plotly/dash/pull/2734) Configure CI for Python 3.10 [#1863](https://github.com/plotly/dash/issues/1863)
+- [#2735](https://github.com/plotly/dash/pull/2735) Configure CI for Python 3.8 and 3.12, drop support for Python 3.6 and Python 3.7 [#2736](https://github.com/plotly/dash/issues/2736)
 
 ## [2.15.0] - 2024-01-31