Updated CHANGES.txt for 3.15.3 release

CHANGES.txt

@@ -1,3 +1,8 @@
+2021-02-25 version 3.15.3 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
+
+  Ruby
+  * Ruby <2.7 now uses WeakMap too, which prevents memory leaks. (#8341)
+
 2021-02-23 version 3.15.2 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
 
   Ruby