BUG #19097: System catalog modifications are allowed by alter

The following bug has been logged on the website:

Bug reference:      19097
Logged by:          RekGRpth
Email address:      rekgrpth@gmail.com
PostgreSQL version: 18.0
Operating system:   docker alpine
Description:

```sql
create table pg_catalog.t (i int);
ERROR:  permission denied to create "pg_catalog.t"
DETAIL:  System catalog modifications are currently disallowed.

create table t (i int);
CREATE TABLE

alter table t set schema pg_catalog;
ALTER TABLE

\dS+ t
                                          Table "pg_catalog.t"
 Column |  Type   | Collation | Nullable | Default | Storage | Compression |
Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
 i      | integer |           |          |         | plain   |             |
|
Replica Identity: NOTHING
Access method: heap

drop table pg_catalog.t;
DROP TABLE
```