Reset waitStart when a lock wait fails

Hi,

Alex and Sami noticed in [1] that RemoveFromWaitQueue() does not clear
PGPROC->waitStart. After lock_timeout, cancel or deadlock the old value
stays until the next lock wait.

It leaks into pg_locks for a short time. The next wait joins the queue
before ProcSleep() stores the new waitStart, and in that window pg_locks
shows the old start time instead of NULL. Backend stopped at ProcSleep()
entry, a few seconds after a lock_timeout:

pid | relation | mode | granted | waitstart
-------+----------+---------------------+---------+-------------------------------
42442 | t | AccessExclusiveLock | f | 2026-09-23
21:01:39.274763-04

With the patch it reads NULL. The patch clears it the same way
ProcWakeup() does, like 70f470314cb did for the grant path. It applies
to 14 and up.

[1]
https://postgr.es/m/CAPrb+Q+XN=sNusXiUeWmMo2H7Qgq3Y4uPekSSLkHcnCyf7GhXg@mail.gmail.com

Thanks,
Shihao