PG has a special type REFCURSOR, which is basically a string, but is used to hold the name of a PLpgSQL cursor. While it's generally used as a variable in a PLpgSQL routine, it can also be used in other contexts (e.g. function parameter or return type, table column type etc.). The type isn't recognized by previous versions of CRDB, so we'll have to add validation to ensure that statements that use REFCURSOR fail until the cluster is fully upgraded to 23.2.

Jira issue: CRDB-31968