[PATCH] Error out if SKIP LOCKED and WITH TIES are both specified

Both bugs #16676[1] and #17141[2] illustrate that the combination of SKIP LOCKED and FETCH FIRST
WITH TIES break expectations when it comes to rows returned to other sessions accessing the same
row.  Since this situation is detectable from the syntax and hard to fix otherwise, forbid for now,
with the potential to fix in the future.

[1] https://www.postgresql.org/message-id/16676-fd62c3c835880da6%40postgresql.org
[2] https://www.postgresql.org/message-id/17141-913d78b9675aac8e%40postgresql.org