BUG #17127: drop column cann't delete from pg_attribute, so it will up to 1600 limits soon

The following bug has been logged on the website:

Bug reference: 17127
Logged by: Zhou Digoal
Email address: digoal(at)126(dot)com
PostgreSQL version: 14beta2
Operating system: CentOS 7.7 x64
Description:

HI, postgresql drop column cann't delete from pg_attribute, so it will up to
1600 limits soon when add and drop column frequenc.

```

do language plpgsql $$
declare
sql text := 'create table c (';
begin
for i in 1..1600 loop
sql := sql||'c'||i||' int8,';
end loop;
sql := rtrim(sql,',')||')';
execute sql;
end;
$$;

postgres=# alter table c add column c int8;
ERROR: 54011: tables can have at most 1600 columns
LOCATION: ATExecAddColumn, tablecmds.c:6573

postgres=# alter table c drop column c1;
ALTER TABLE
postgres=# alter table c add column c int8;
ERROR: 54011: tables can have at most 1600 columns
LOCATION: ATExecAddColumn, tablecmds.c:6573

postgres=# vacuum full c;
VACUUM
postgres=# alter table c add column c int8;
ERROR: 54011: tables can have at most 1600 columns
LOCATION: ATExecAddColumn, tablecmds.c:6573

postgres=# select attname from pg_attribute where attrelid ='c'::regclass
and attisdropped;
attname
------------------------------
........pg.dropped.1........
(1 row)
```

best regards, digoal