databases/postgresql15-*: Update to 15.0

The PostgreSQL Global Development Group today announced the release of
PostgreSQL 15, the latest version of the world’s most advanced open
source database.

PostgreSQL 15 builds on the performance improvements of recent releases
with noticeable gains for managing workloads in both local and
distributed deployments, including improved sorting. This release
improves the developer experience with the addition of the popular MERGE
command, and adds more capabilities for observing the state of the
database.

Release notes: https://www.postgresql.org/about/news/postgresql-15-released-2526/