pg_createsubscriber: Obstruct SQL injection via subscription names.

pg_createsubscriber: Obstruct SQL injection via subscription names.

drop_existing_subscription() neglected to escape the subscription
name when generating its query string.  To fix, use
PQescapeIdentifier() to construct a properly escaped name, and use
it in the ALTER SUBSCRIPTION and DROP SUBSCRIPTION commands.

Reported-by: Yu Kunpeng <yu443940816@live.com>
Author: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Security: CVE-2026-6476
Backpatch-through: 17