Restore lock level to set vacuum flags

Commit 27838981 mistakenly reduced the lock level from exclusive to
shared that is acquired to set PGPROC->statusFlags; this was reverted
by dcfff74f, but failed to do so in one spot.  Fix it.

Backpatch to 14.

Noted by Andres Freund.

Discussion: https://postgr.es/m/20211111020724.ggsfhcq3krq5r4hb@alap3.anarazel.de

src/backend/replication/slot.c

@@ -511,7 +511,7 @@ ReplicationSlotRelease(void)
 	MyReplicationSlot = NULL;
 
 	/* might not have been set when we've been a plain slot */
-	LWLockAcquire(ProcArrayLock, LW_SHARED);
+	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 	MyProc->statusFlags &= ~PROC_IN_LOGICAL_DECODING;
 	ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
 	LWLockRelease(ProcArrayLock);