Mattermost 11 drops MySQL support

[girish]

Mattermost 11 has now completely removed MySQL support. https://mattermost.com/blog/mattermost-postgresql-support-policy/ and https://github.com/mattermost/docs/issues/8202

Cloudron Mattermost package is already using PostgreSQL since a while now. But we have been pushing updates to the MySQL package in the background. We will stopping updates for the MySQL package from here on.

Migration guide - https://docs.cloudron.io/packages/mattermost/#migrate-to-postgres