Added changelog entry for removing binary gems for Ruby <2.5. (#7454)

CHANGES.txt

@@ -78,6 +78,7 @@
 
   Ruby
   * [experimental] Implemented proto3 presence for Ruby. (#7406)
+  * Stop building binary gems for ruby <2.5 (#7453)
   * Fix for wrappers with a zero value (#7195)
   * Fix for JSON serialization of 0/empty-valued wrapper types (#7198)
   * Call "Class#new" over rb_class_new_instance in decoding (#7352)