build: set release date for 2.4.0

Author: rouilj

CHANGES.txt

@@ -12,7 +12,7 @@ v2.7.2 or later are required to run newer releases of Roundup.
 Roundup 2.0 supports Python 3.4 and later. Roundup 2.1.0 supports
 python 3.6 or newer (3.4/3.5 might work, but they are not tested).
 
-2024-XX-YY 2.4.0
+2024-07-13 2.4.0
 
 Fixed: