Description of problem:

When upgrading from Satellite 6.15 to 6.16, The PostgreSQL database would be upgraded from 12 to 13 as well. This requires additional disk space in /var/lib/pgsql and the check related to the same is present in satellite-maintain  https://github.com/theforeman/foreman_maintain/blob/master/definitions/checks/disk/available_space_postgresql13.rb as well. 

But It needs to be part of the upgrade prerequisites as well i.e. in the Connected\Disconnected Satellite Upgrade guide for 6.16, under the `Satellite Server upgrade considerations` part, We need to document this change in the PostgreSQL version and how much additional free disk space would be required in /var/lib/pgsql 

Once the documentation has it, The same prerequisite can be documented in the Satellite Upgrade Helper portal app as well via a different Jira ( which i or AshishH can take care of ).