postgresql: update to 18.3

postgresql: update to 18.3

Major version update from 17.5 to 18.3.

PostgreSQL 18 (released September 2025) brings:
 * Asynchronous I/O (AIO) for shared buffers, sequential scans,
   bitmap heap scans and pg_prewarm.
 * Skip scans for B-tree indexes.
 * Performance improvements for partition pruning.
 * Logical replication: improved replication of generated columns,
   protocol version 5.
 * Native UUIDv7 support.
 * Larger I/O for sequential and parallel scans.
 * Concurrent reindex of partitioned tables.
 * pg_dump: --filter for selective dumps.
 * Numerous SQL/JSON improvements.
 * New built-in role pg_signal_autovacuum_worker.

18.3 is the third maintenance release with bug fixes since 18.0.

Drop the now obsolete pg_config_ext.h copy in Build/InstallDev: this
header has been removed upstream in PostgreSQL 18.

Link: https://www.postgresql.org/docs/release/18.0/
Link: https://www.postgresql.org/docs/release/18.3/
Signed-off-by: Daniel Golle <redacted>