Merge branch 'mm-6.1' into 'master'

Update doc to sync gitlab release with mattermost release

See merge request gitlab-org/gitlab!75231

doc/integration/mattermost/index.md

@@ -339,6 +339,11 @@ Below is a list of Mattermost versions for GitLab 11.10 and later:
 | 14.2 | 5.37 |
 | 14.3 | 5.38 |
 | 14.4 | 5.39 |
+| 14.5 | 5.39 |
+| 14.6 | 6.1  |
+
+- GitLab 14.5 remained on Mattermost 5.39
+- GitLab 14.6 updates to Mattermost 6.1 instead of 6.0
 
 NOTE:
 When upgrading the Mattermost version, it is essential to check the
@@ -364,6 +369,21 @@ If this is not the case, there are two options:
 
 For a complete list of upgrade notices and special considerations for older versions, see the [Mattermost documentation](https://docs.mattermost.com/administration/important-upgrade-notes.html).
 
+## Upgrading GitLab Mattermost to 14.6
+
+GitLab 14.6 includes Mattermost 6.1, and also includes the migrations for Mattermost 6.0. For information about upgrading and for ways to reduce the downtime of those migrations, read the [Important Upgrade Notes](https://docs.mattermost.com/administration/important-upgrade-notes.html) for both versions.
+
+NOTE:
+The Mattermost upgrade notes refer to different impacts when used with a PostgreSQL versus a MySQL database. The GitLab Mattermost included with the GitLab Linux packages uses a PostgreSQL database.
+
+If you need to connect in the database to perform any manual migrations, run the following:
+
+```console
+sudo gitlab-psql -d mattermost_production
+```
+
+You can then run the necessary queries that are described in the [Important Upgrade Notes](https://docs.mattermost.com/administration/important-upgrade-notes.html).
+
 ## Upgrading GitLab Mattermost from versions prior to 11.0
 
 With version 11.0, GitLab introduced breaking changes which affected Mattermost configuration.