After upgrade Postgres python is the same version

I upgraded my postgres from 13.5 to 16.2 version (and the RHEL from 7.5 to 8.9). The problem is, python was not upgraded. I created procedure pyver():

CREATE OR REPLACE FUNCTION pyver ()
RETURNS TEXT
AS $$
    import sys
    pyversion = sys.version
    return pyversion
$$ LANGUAGE 'plpython3u';

For show of python version used by Postgres. When I run it on Python 13.5, I get this result:

# psql -d database
psql (13.5)
Type "help" for help.

postgres@database # select pyver();
                  pyver
-----------------------------------------
 3.6.8 (default, Aug 13 2020, 07:36:02) +
 [GCC 4.8.5 20150623 (Red Hat 4.8.5-39)]
(1 řádka)

postgres@database #

And when I run it on upgraded DB, I get this result:

# psql -d database
psql (16.2)
Type "help" for help.

database=# select pyver();
                  pyver
-----------------------------------------
 3.6.8 (default, Jan  5 2024, 09:14:44) +
 [GCC 8.5.0 20210514 (Red Hat 8.5.0-20)]
(1 row)

database=#

On the new RedHat is Python 3.9.18 installed. How do I get postgres to use a newer version of python? Upgrade was made by this way: - was installed new server with RedHat 8.9 (Python 3.9) - was installed Postgres binaries (version 13.5) and created user postgres - then was connected disk with Postgres databases (version 13.5) from old server - the DB's was started on new server - next step was installation of Postgres 16.2 - then was start with upgrade of DB's to new Postgres version via this command:

time /usr/pgsql-16/bin/pg_upgrade --jobs=16 -d /postgres/pgsql/database/data13 -D /postgres/pgsql/database/data16 -b /usr/pgsql-13/bin/ -B /usr/pgsql-16/bin/ --link

- after upgrade runs DB's on new version And now we find the proglem with Python. The main DB has 70TB size. Thanks Michal