Comment # 1 on bug 1175643 from Reinhard Max

We maintain PostgreSQL through the update-alternatives mechanism and the
symlink pointing from /usr/bin/pg_config to the respective
/usr/lib/postgresqlXX/bin/pg_config file will only be created for the
PostgreSQL version that is currently "active", and only if the server-devel
package of that version is installed.

In order to get the pg_config symlink pointing to the postgresql11 version you
either need to uninstall all newer PostgreSQL packages (i.e. postgresql12*), so
that postgresql11* becomes the default automatically, or you can manually set
the default version to postgresql11:

# update-alternatives --set postgresql /usr/lib/postgresql11