discourse core does not use the vector extension, so the pgvector image is
unnecessary. Switch db to pgautoupgrade/pgautoupgrade:17-trixie, which handles
major-version upgrades (old binaries, initdb, pg_upgrade) itself.
pgautoupgrade assumes the old cluster's install user (oid 10) equals
POSTGRES_USER and fails pg_upgrade otherwise (issue #115). A thin wrapper
entrypoint detects the old install user when an upgrade is pending and exports
POSTGRES_USER so pgautoupgrade matches it. Replaces the 64-line hand-rolled
pg_upgrade entrypoint.
