databases/postgresql15-server: update to 15beta5

This release contains previews of all features that will be available
when PostgreSQL 15 is made generally available, though some details of
the release can change during the beta period.

To upgrade to PostgreSQL 15 Beta 4 from an earlier beta or previous
version of PostgreSQL, you will need to use a strategy similar to
upgrading between major versions of PostgreSQL (e.g. pg_upgrade or
pg_dump / pg_restore). For more information, please visit the
documentation section on upgrading.

URL: https://www.postgresql.org/about/news/postgresql-15-beta-4-released-2507/
Release notes: https://www.postgresql.org/docs/15/release-15.html