[improvement](scan_queue_mem_limit) scan queue mem limit is so small for (#24553)

a wide table

Users rarely set scan_queue_mem_limit, so it almost often works as 2G/20. However,
somecases we need set it to a larger value, especially for insrt into
select from a wide table.

fe/fe-core/src/main/java/org/apache/doris/qe/SessionVariable.java

@@ -1551,7 +1551,7 @@ public class SessionVariable implements Serializable, Writable {
     }
 
     public void setMaxScanQueueMemByte(long scanQueueMemByte) {
-        this.maxScanQueueMemByte = Math.min(scanQueueMemByte, maxExecMemByte / 20);
+        this.maxScanQueueMemByte = Math.min(scanQueueMemByte, maxExecMemByte / 2);
     }
 
     public boolean isSqlQuoteShowCreate() {