Add RubyConf China 2024 announced_on key

Author: marcoroth

_data/conferences.yml

@@ -2848,6 +2848,7 @@
   start_date: 2024-11-30
   end_date: 2024-12-01
   url: https://www.rubyconfchina.org
+  announced_on: 2024-07-17
 
 - name: RubyWorld Conference 2024
   location: Matsue, Japan