sql: multiple FK constraints on the same column are erroneously allowed in 19.1

[thoszhang]

In 19.1, and possibly earlier versions, we accidentally neglected the primary key when checking all indexes to ensure that no column is involved in more than one foreign key constraint (on the referencing side). There's no specific incorrect behavior expected from this, but it still leads to confusing results, as in #42400.

See #37590 (comment) for another example.

This was fixed incidentally by #37433 in 19.2, but we can backport a fix to 19.1.

[thoszhang]

Closing - we're not going to "fix" this in past versions, see #42400 for more context