The PostgreSQL version 11 is obsolete, but the server or client packages are still installed. Please install the latest packages (postgresql-13 and postgresql-client-13) and upgrade the existing  clusters with pg_upgradecluster (see manpage).

Please be aware that the installation of postgresql-13 will automatically create a default cluster 13/main. If you want to upgrade the 11/main cluster, you need to remove the already existing 13 cluster (pg_dropcluster --stop 13 main, see manpage for details).

Please see /usr/share/doc/postgresql-common/README.Debian.gz for details.