Update changelog for 16.8.4

[ci skip]

Author: GitLab Release Tools Bot

CHANGELOG.md

@@ -57,6 +57,12 @@ No changes.
 - [Update PostgreSQL 13 and 14](gitlab-org/omnibus-gitlab@8d6a65a402e0aa6fc7f826c424cb778f3e0d8b79)
 - [Update libxml2 from 2.10.4 to 2.12.3](gitlab-org/omnibus-gitlab@18f0dcced0f8597992b87bb0b01ae5d9f782842f)
 
+## 16.8.4 (2024-03-06)
+
+### Security (1 change)
+
+- [Mattermost Security Updates February 14, 2024](gitlab-org/security/omnibus-gitlab@44a51aa75c9e1c3e06cc0e6d283112ac8c76a37d) ([merge request](gitlab-org/security/omnibus-gitlab!418))
+
 ## 16.8.3 (2024-02-20)
 
 No changes.