feat: add POSTGRES_18 to DatabaseVersion (#32335)

* feat: add POSTGRES_18 to DatabaseVersion

PiperOrigin-RevId: 852794094

Source-Link: googleapis/googleapis@a745ee4

Source-Link: googleapis/googleapis-gen@51bf5e1
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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: gcf-owl-bot[bot]