Add Mattermost changelog entry for version 7.1

• Release post: https://mattermost.com/blog/mattermost-v7-1-is-now-available/

docs/software/mattermost/changelog.md

@@ -2,6 +2,11 @@
 title: Service Changelog
 ---
 
+## 2022-08-25
+
+* Update Mattermost to version 7.1 -
+  [Release Post](https://mattermost.com/blog/mattermost-v7-1-is-now-available/)
+
 ## 2022-07-27
 
 * Update Mattermost to version 7.0 -