With only Puma carried in GitLab 14.0 onwards
there is no possibility of minimizing any downtime
in single-node upgrades.

When the migration is run while leaving
the previous version's puma running, the
database changes are picked up in Rails
dynamically and UI errors begin to be
thrown to users.

Given the disruption of features is essentially
the same to users as a downtime when it
affects critical features such as merge requests
the section is false in indicating that
downtime is minimized.

Additionally this section is often discovered
by users and blindly followed even for
multiple version jumps that leads to
broken upgrades.

This change removes all sections related to zero
downtime upgrade instructions of single nodes.