Remove references to discontinued Mattermost communication channel
Mattermost is not used anymore as a communication channel, remove references and broken links from navigation, contact and some documentation pages.
This instance will be upgraded to Heptapod 1.5.0rc2 (GitLab 16.11.1) on 2024-05-10 at 18:00 UTC+2
Mattermost is not used anymore as a communication channel, remove references and broken links from navigation, contact and some documentation pages.