Update mattermost from V9 to V10

[rahulporuri]

Ref changelog for V10 - https://docs.mattermost.com/about/mattermost-v10-changelog.html

[rahulporuri]

At the very least, we need to upgrade to version 9.11 from 9.10 as 9.10 is nearing End of Life and 9.11 is an Extended Support Release version. Ref https://docs.mattermost.com/about/release-policy.html#extended-support-releases