PG17 compatibility: fix pg16 to pg17 upgrade

[colm-mchugh]

In citus_prepapre_upgrade(), don't drop any_value when upgrading from PG16+, because PG16+ has its own any_value function. Attempting to do so results in the error seen in pg16-pg17 upgrade:

ERROR:  cannot drop function any_value(anyelement) because it is required by the database system
CONTEXT:  SQL statement "DROP AGGREGATE IF EXISTS pg_catalog.any_value(anyelement)"

When 16 becomes the minimum supported Postgres version, the drop statements can be removed.

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[naisila]

Thanks for the fix, I remember I have implemented this so sorry for that mistake :)

However we can't merge as it is as we need to create a new .sql file for the new citus version. We don't modify old .sql files. I don't remember where the ocumentation for this part is, but basically we need to add a new 13.0-1.sql file with the modified script, and also an upgrade and downgrade path for Citus https://github.com/citusdata/citus/tree/release-13.0/src/backend/distributed/sql

[colm-mchugh]

However we can't merge as it is as we need to create a new .sql file for the new citus version. We don't modify old .sql files. I don't remember where the ocumentation for this part is, but basically we need to add a new 13.0-1.sql file with the modified script, and also an upgrade and downgrade path for Citus https://github.com/citusdata/citus/tree/release-13.0/src/backend/distributed/sql

The latest push reverts the modification to the existing citus sql file and adds a new one; however it probably needs #7792 to enable the pg16 -> pg17 upgrade to pass (?)