Fix typo in GetRunningTransactionData()

e85662df44 made GetRunningTransactionData() calculate the oldest running
transaction id within the current database.  However, because of the typo,
the new code uses oldestRunningXid instead of oldestDatabaseRunningXid
in comparison before updating oldestDatabaseRunningXid.  This commit fixes
that issue.

Reported-by: Noah Misch
Discussion: https://postgr.es/m/20240630231816.bf.nmisch%40google.com
Backpatch-through: 17

src/backend/storage/ipc/procarray.c

@@ -2779,7 +2779,7 @@ GetRunningTransactionData(void)
 		 * Also, update the oldest running xid within the current database.
 		 */
 		if (proc->databaseId == MyDatabaseId &&
-			TransactionIdPrecedes(xid, oldestRunningXid))
+			TransactionIdPrecedes(xid, oldestDatabaseRunningXid))
 			oldestDatabaseRunningXid = xid;
 
 		if (ProcGlobal->subxidStates[index].overflowed)