- pg_backup.sh: use the db service's POSTGRES_USER (default postgres) for the
dump/drop/recreate instead of detecting the superuser at runtime, since the
recipe now sets that env var; one-line the constants comment
- release note: explain the in-place pg_upgrade + the POSTGRES_USER override
- bump PG_BACKUP_VERSION v4
Verified on cctest: backup + restore via the hooks round-trips with POSTGRES_USER.