Apply August Mattermost Security Updates

Addresses a medium level vulnerability discovered during a security research review by severus.

https://mattermost.com/blog/mattermost-security-update-5-13-3-5-12-6-5-9-4-esr-released/

There is also an update we missed for the release included in 12.0 that we should apply if we do another 12.0 security release:

This security update addresses a medium-level vulnerability discovered during a security research review by Zonduu.

https://mattermost.com/blog/mattermost-security-update-5-11-1-5-10-2-5-9-2-4-10-10-esr-released/

Dev security issue: https://dev.gitlab.org/gitlab/omnibus-gitlab/issues/42