pgsql: Replace last PushOverrideSearchPath() call with set_config_optio

Replace last PushOverrideSearchPath() call with set_config_option().

The two methods don't cooperate, so set_config_option("search_path",
...) has been ineffective under non-empty overrideStack.  This defect
enabled an attacker having database-level CREATE privilege to execute
arbitrary code as the bootstrap superuser.  While that particular attack
requires v13+ for the trusted extension attribute, other attacks are
feasible in all supported versions.

Standardize on the combination of NewGUCNestLevel() and
set_config_option("search_path", ...).  It is newer than
PushOverrideSearchPath(), more-prevalent, and has no known
disadvantages.  The "override" mechanism remains for now, for
compatibility with out-of-tree code.  Users should update such code,
which likely suffers from the same sort of vulnerability closed here.
Back-patch to v11 (all supported versions).

Alexander Lakhin.  Reported by Alexander Lakhin.

Security: CVE-2023-2454

Branch
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REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/78119a0bf98edb13e7f79c6402378376cc6b6ca4

Modified Files
--------------
src/backend/catalog/namespace.c         |  4 +++
src/backend/commands/schemacmds.c       | 37 +++++++++++++++++++--------
src/test/regress/expected/namespace.out | 45 +++++++++++++++++++++++++++++++++
src/test/regress/sql/namespace.sql      | 24 ++++++++++++++++++
4 files changed, 100 insertions(+), 10 deletions(-)