Update Omnibus Mattermost versions

Omnibus 16.9 ships with Mattermost 9.4.

Omnibus MR: https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/7384

doc/integration/mattermost/index.md

@@ -359,6 +359,7 @@ Below is a list of Mattermost version changes for GitLab 14.0 and later:
 
 | GitLab version | Mattermost version | Notes                                                                                    |
 | :------------- | :----------------- | ---------------------------------------------------------------------------------------- |
+| 16.9           | 9.4                |                                                                                          |
 | 16.7           | 9.3                |                                                                                          |
 | 16.6           | 9.1                |                                                                                          |
 | 16.5           | 9.0                |                                                                                          |