Update Mattermost to 5.6.2
Mattermost 5.6 contains a number of smaller UI features and new CLI commands. It also removes the deprecated WebRTC feature and support for small screen sizes in IE11. The full changelog it is available here. 5.6.1 and 5.6.2 only contain non-security bug fixes, so they don't contain anything that will be backported to older versions of Mattermost.