Update Mattermost version

data/release_posts/unreleased/mattermost.yml

+# Omnibus and performance (required)
+features: 
+  secondary: 
+    - name: "Omnibus improvements"
+      available_in: [core, starter, premium, ultimate]
+      gitlab_com: false
+      documentation_link: 'https://docs.gitlab.com/omnibus/'
+      reporter: ljlane
+      description: |
+        - GitLab 12.6 includes Mattermost 5.17.1, an [open source
+          Slack-alternative](https://mattermost.com/). This version of Mattermost
+          is focused on [quality
+          improvements](https://docs.mattermost.com/administration/changelog.html#release-v5-17-1-quality-release).