Notes on upgrading from Oracle 12c to 19c

Technically, we don't need to document how to upgrade from Oracle 12c to Oracle 19c – that's what the Oracle documentation is for. But it is possible that QE testing might reveal some points about this upgrade procedure that are of particular importance for 3scale and/or some kinds of issues that affect the upgrade.

This doc issue is a placeholder to remind us to follow up with the QE team to find out if there are any notes we should add to the 3scale docs relating to this procedure.