Current behaviour

When performing an upgrade from 2.11 to 2.12 the documentation states that only Postgres 13 is supported when using an external DB. There is no possibility to remain in a supported configuration when performing the upgrade due to this limitation and in the event of a failed upgrade the unsupported configuration may not be able to be rolled back.

Expected behaviour

When Postgres is used as an external database for system then both versions 10 & 13 are supported to facilitate the upgrade procedure and provide some safety in the event of a failed upgrade of 3scale (the external DB might now be "stuck" on Postgres 13 until a successful upgrade of 3scale can be complete.

Regarding the external DB this is only relevant for the system DB, it is not expected to cover scenarios where the zync-db has also been externalised.