Upgrade to 15

During updating to Debian 12, installer left postgresql-13 with info to manually upgrade.

apt install postgresql-15
pg_dropcluster --stop 15 main
pg_upgradecluster 13 main
# OR if upgrading from Debian 10
# pg_upgradecluster 11 main
 
Success. Please check that the upgraded cluster works. If it does,
you can remove the old cluster with
    pg_dropcluster 11 main