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(at)gmail(dot)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
```