Praefect: Enable SQL failover by default

Now Praefect has the ability to disable mutation on repositories when these are out of date and failover happens, data loss should not happen anymore.

As such, this will now be the default, after: gitaly!2218 (merged)

This change updates the omnibus defaults to reflect this, but is not blocked on the upstream merge request.

Part of: gitaly#2682 (closed)