Summary:
The test query is a UNION of two SELECT <constant> statements, and
EXPLAIN deparses the column references, such as keys in the ORDER BY,
as the constant values. Apparently pg11 picks the value from the last
UNION'ed statement, while pg15 picks first.

It looks like we can't do anything about that without interfering
with the test's objective, so just update the expected results in the
pg15-based branch.
Jira: DB-13181

Test Plan: ./yb_build.sh --java-test 'org.yb.pgsql.TestPgRegressParallel#testPgRegressParallel'

Reviewers: mtakahara, telgersma

Reviewed By: telgersma

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D38944