chore: update changelog

Author: nbacquey

cardano-testnet/CHANGELOG.md

@@ -5,6 +5,7 @@
 * [Fix discrepancy in security parameter between Byron and Shelley genesis files](https://github.com/IntersectMBO/cardano-node/pull/6188)
 * [Add an option to dump/load configuration sandbox](https://github.com/IntersectMBO/cardano-node/pull/6239)
 * [Add flag to support P2P topology](https://github.com/IntersectMBO/cardano-node/pull/6263)
+* [Add flag to update time stamps in custom environment](https://github.com/IntersectMBO/cardano-node/pull/6275)
 
 ## 10.0.0