Use "concurrent delete" in serialization error for TM_Deleted cases

Use "concurrent delete" in serialization error for TM_Deleted cases

In ExecLockRows() and ri_LockPKTuple(), the TM_Deleted code path was
using the same "could not serialize access due to concurrent update"
message as the TM_Updated path.  Use "concurrent delete" instead, since
the tuple was deleted, not updated.  The ExecLockRows() instance was
likely a copy-paste error per Andres; the ri_LockPKTuple() instance
was carried over from the same pattern in commit 2da86c1ef9.

Update affected isolation test expected files accordingly and add
a new test to fk-concurrent-pk-upd.spec with concurrent delete of the
PK row.

The ExecLockRows() change is master-only for lack of user complaints
and to avoid breaking anything that might match on the error text.

Reported-by: Jian He <jian.universality@gmail.com>
Author: Amit Langote <amitlangote09@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://postgr.es/m/CACJufxEG1JTCq4A1gnNAu-bGAq9Xn=Xkf7kC3TRWFz6iuUOuRA@mail.gmail.com