EXPERIMENT: probe PG18 availability in pgxn-tools image (bump NEWEST 17->18)

Author: jnasbyupgrade

.github/workflows/ci.yml

@@ -140,7 +140,7 @@ jobs:
           # `test` and `pg-upgrade-test` matrices below both derive their
           # version lists from here, so they cannot silently drift onto
           # different lists. To add or drop a major, edit only NEWEST/FLOOR.
-          NEWEST=17
+          NEWEST=18
           FLOOR=10
           supported=$(seq "$NEWEST" -1 "$FLOOR")
           # Emit a JSON array from a list of ints, for matrix: to consume