pgsql: Fix error message for ALTER CONSTRAINT ... NOT VALID

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

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e16c9cd331314fcf3f7a8d9e3e20aaee448162e4

Modified Files
--------------
src/backend/parser/gram.y                 | 6 ++++++
src/test/regress/expected/constraints.out | 5 +++++
src/test/regress/expected/foreign_key.out | 2 +-
src/test/regress/sql/constraints.sql      | 3 +++
4 files changed, 15 insertions(+), 1 deletion(-)