Use v2.4 as old for comparing against latest (#3181)

This is regarding donate-cpu functionality.

Author: bolry

tools/donate-cpu-server.py

@@ -27,7 +27,7 @@
 # changes)
 SERVER_VERSION = "1.3.16"
 
-OLD_VERSION = '2.3'
+OLD_VERSION = '2.4'
 
 
 # Set up logging