Add note about data loss during Postgres downgrade

This is in reference to: gitlab#216029 (closed)

Downgrading one's bundled database is a destructive action and with the move to no longer support Postgres <11 in GitLab 13.0 we need to provide a very clear warning in the event people try to do incremental upgrades and run into database issues. We had a related incident this morning and we noticed that the docs should really include this information.