Fix error message for ALTER CONSTRAINT ... NOT VALID

Álvaro Herrera <alvherre@kurilemu.de>

Commit: c989affb527d330898e92e6223e2218e702176ed
Author: Álvaro Herrera <alvherre@kurilemu.de>
Date: 2025-07-02T15:02:27Z
Fix error message for ALTER CONSTRAINT ... NOT VALID

Trying to alter a constraint so that it becomes NOT VALID results in an
error that assumes the constraint is a foreign key.  This is potentially
wrong, so give a more generic error message.

While at it, give CREATE CONSTRAINT TRIGGER a better error message as
well.

Co-authored-by: jian he <jian.universality@gmail.com>
Co-authored-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Co-authored-by: Álvaro Herrera <alvherre@kurilemu.de>
Co-authored-by: Amul Sul <sulamul@gmail.com>
Discussion: https://postgr.es/m/CACJufxHSp2puxP=q8ZtUGL1F+heapnzqFBZy5ZNGUjUgwjBqTQ@mail.gmail.com

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