BUG #18660: information_schema.columns.ordinal_position has gaps when primary key columns are dropped

The following bug has been logged on the website:

Bug reference:      18660
Logged by:          Lukas Eder
Email address:      lukas.eder@gmail.com
PostgreSQL version: 17.0
Operating system:   Linux (in Docker on Windows)
Description:

The problem can be illustrated with this simple reproducer script:

CREATE TABLE t (
  i1 INT NOT NULL,
  i2 INT NOT NULL,
  t TEXT NOT NULL
);

ALTER TABLE t ADD PRIMARY KEY (i1, i2);
ALTER TABLE t DROP COLUMN i1;
ALTER TABLE t ADD COLUMN i3 INT NOT NULL;

SELECT column_name, ordinal_position
FROM information_schema.columns
WHERE table_name = 't'
ORDER BY ordinal_position;

The query results in:

|column_name|ordinal_position|
|-----------|----------------|
|i2         |2               |
|t          |3               |
|i3         |4               |

This is against the SQL standard specification of the
information_schema.columns.ordinal_position column, which has a constraint
as follows:

CONSTRAINT COLUMNS_ORDINAL_POSITION_CONTIGUOUS_CHECK CHECK (0 = ALL (
  SELECT MAX(ORDINAL_POSITION) - COUNT(*)
  FROM COLUMNS
  GROUP BY
  TABLE_CATALOG, TABLE_SCHEMA, TABLE_NAME 
))

I suspect this is because the internal pg_attribute.attnum leaks into the
information_schema.columns.ordinal_position:

SELECT a.attname, a.attnum
FROM pg_attribute a
JOIN pg_class c ON a.attrelid = c.oid
WHERE c.relname = 't'
AND attnum > 0
ORDER BY attnum;

Produces this:

|attname                     |attnum|
|----------------------------|------|
|........pg.dropped.1........|1     |
|i2                          |2     |
|t                           |3     |
|i3                          |4     |