3.13.1 release notes: Cowboy was also bumped

Author: michaelklishin

release-notes/3.13.1.md

@@ -153,6 +153,7 @@ Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://
 
  * `osiris` was updated to [`1.8.1`](https://github.com/rabbitmq/osiris/releases)
  * `khepri` was upgraded to [`0.13.0`](https://github.com/rabbitmq/khepri/releases)
+ * `cowboy` was updated to [`2.12.0`](https://ninenines.eu/docs/en/cowboy/2.12/guide/migrating_from_2.11/)
 
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