Update changelog for 15.11.6

[ci skip]

Author: GitLab Release Tools Bot

CHANGELOG.md

@@ -74,6 +74,10 @@ No changes.
 - [Drop bundler software definition](gitlab-org/omnibus-gitlab@3f08274227a17adf5f7d42403804af198895a726) ([merge request](gitlab-org/omnibus-gitlab!6849))
 - [Update Mattermost to 7.10.0](gitlab-org/omnibus-gitlab@ec498e8b899bfaf00d07d161a5a03ae9aed66612) by @akis.maziotis ([merge request](gitlab-org/omnibus-gitlab!6827))
 
+## 15.11.6 (2023-05-24)
+
+No changes.
+
 ## 15.11.5 (2023-05-19)
 
 No changes.