Stabilize multi_insert_select expected: accept unqualified columns in WHERE … IS NOT NULL (PG15–PG18) (#8139)

DESCRIPTION: Stabilize multi_insert_select expected: accept unqualified
columns in WHERE … IS NOT NULL

fixes #8133 


**Context**

* With PG18, ruleutils adds a GROUP RTE and improves column-name dedup.
As a side-effect, Vars that point at the GROUP RTE print as unqualified
column names even when `varprefix` is true.
* In Citus’ vendored `ruleutils_18.c` we already flattened GROUP Vars in
`targetList` and `havingQual`, but not in `jointree->quals`.
* For queries like `INSERT … SELECT … GROUP BY …`, Citus injects an
implicit null-guard on the group key in the WHERE clause. Because that
Var was still referencing the GROUP RTE, the deparser emitted `WHERE
(user_id IS NOT NULL)` instead of `WHERE (raw_events_first.user_id IS
NOT NULL)`, causing regress diffs only in grouped SELECTs.
* Related upstream change: PostgreSQL commit
`52c707483ce4d0161127e4958d981d1b5655865e` (ruleutils column-name de-dup
/ GROUP RTE exposure).

**What changed**

* Added an alternative expected file
`src/test/regress/expected/multi_insert_select_0.out` to keep CI green
across mixed environments where the qualified form may still be
produced.

Author: m3hm3t