UPDATE pg_catalog.pg_class as NO Superuser??

Dear list,

on my system I have multiple user.

I want that the user xy (who is no superuser) can Update a systemtable with:

UPDATE pg_catalog.pg_class SET reltriggers = 0;

With psql I already wrote:

ALTER TABLE pg_catalog.pg_class OWNER TO xy;

and

GRANT ALL PRIVILEGES ON pg_catalog.pg_class TO xy;

I check it with \dS
and see:

List of relations
Schema | Name | Type | Owner
------------+--------------------------+-------+-----------
pg_catalog | pg_aggregate | table | postgres
pg_catalog | pg_am | table | postgres
pg_catalog | pg_amop | table | postgres
pg_catalog | pg_amproc | table | postgres
pg_catalog | pg_attrdef | table | postgres
pg_catalog | pg_attribute | table | postgres
pg_catalog | pg_auth_members | table | postgres
pg_catalog | pg_authid | table | postgres
pg_catalog | pg_autovacuum | table | postgres
pg_catalog | pg_cast | table | postgres
pg_catalog | pg_class | table | xy

But always if I sent as user xy the
"UPDATE pg_catalog.pg_class SET reltriggers = 0;"-command I get:

"SQL error:

ERROR: permission denied for relation pg_class"

Why is that? MUST I be a superuser for that?
Is there any way to set that command as user xy?

Thank you very much in advance, Kai

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