Fix broken MySQL schema when upgrading from 1.9.2, 1.9.3 to 1.9.4

As reported by a customer, see support#17175, there is a problem with the MySQL schema upgrade. This has been traced to 9.5 schema version script being edited after it was released.

The following actions should be taken:

document the issue in known issues
provide a script that allows people who already tried to upgrade to fix their DB and continue
alter the 9.4_to_9.5 upgrade script to not introduce the changes
create 9.5_to_9.6 upgrade that introduces the changes properly

Edited Feb 19, 2021 by Andrei Pavel

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