For the records / to have it tracked / as a reference, the following is shown on Debian if having PostgreSQL 14 installed and the system is receiving an upgrade to PostgreSQL 15:
The PostgreSQL version 14 is obsolete, but the server or client packages are still installed. Please install the latest packages (postgresql-15 and postgresql-client-15) and upgrade the existing clusters with pg_upgradecluster (see manpage).
Please be aware that the installation of postgresql-15 will automatically create a default cluster 15/main. If you want to upgrade the 14/main cluster, you need to remove the already existing 15 cluster (pg_dropcluster --stop 15 main, see manpage for details).
The old server and client packages are no longer supported. After the existing clusters are upgraded, the postgresql-14 and postgresql-client-14 packages should be removed.
Please see /usr/share/doc/postgresql-common/README.Debian.gz for details.