Support for PG 18 and MobilityDB 1.3

[estebanzimanyi]

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[estebanzimanyi]

Superseded, both halves.

The src/ changes are on master: vacuum_delay_point(false) in mgistvacuum.c and mspgvacuum.c, and pg_newlocale_from_collation(PG_GET_COLLATION())->collate_is_c in text_mspgist.c. Master carries them together with the read-stream vacuum scan, the support-function renumbering and the amcanhash/amconsistentequality/amconsistentordering handler fields, so this branch's src/ tree is behind master rather than ahead of it.

The contrib/mobilitydb changes state the extract methods against the MobilityDB 1.3 API; #30 states them against 1.4.

The remaining PostgreSQL 18 work is #29, which names GIST_TRANSLATE_CMPTYPE_PROC and numbers the MGiST extract method 13.