Merge branch 'mattermost-5.12' into 'master'

Update Mattermost to 5.12.2

See merge request gitlab-org/omnibus-gitlab!3403

changelogs/unreleased/mattermost-5-12.yml

+---
+title: Update Mattermost to 5.12.2
+merge_request:
+author: Harrison Healey
+type: other

config/software/mattermost.rb

@@ -17,10 +17,10 @@
 #
 name 'mattermost'
-default_version '5.11.1'
+default_version '5.12.2'
 source url: "https://releases.mattermost.com/#{version}/mattermost-team-#{version}-linux-amd64.tar.gz",
-       md5: '9d661f9af12285daef67dea62791eba4'
+       md5: 'c2da6347d8afd2a0732804a63e0f7def'
 relative_path 'mattermost'

doc/gitlab-mattermost/README.md

@@ -263,6 +263,7 @@ Below is a list of Mattermost versions for GitLab 9.0 and later:
 | 11.10 | 5.9 |
 | 11.11 | 5.10 |
 | 12.0 | 5.11 |
+| 12.1 | 5.12 |
 It is possible to skip upgrade versions starting from Mattermost v3.1. For example, Mattermost v3.1.0 in GitLab 8.9 can upgrade directly to Mattermost v3.4.0 in GitLab 8.12.