Update RubyConf Africa 2024 CFP deadline (ruby-conferences#640)

Author: marcoroth

_data/conferences.yml

@@ -2627,7 +2627,7 @@
   url: https://rubyconf.africa/
   twitter: ruby_african
   cfp_open_date: 2024-03-20
-  cfp_close_date: 2024-04-30
+  cfp_close_date: 2024-05-31
   cfp_link: https://www.papercall.io/rubyconfafrica2024
 
 - name: Madison+ Ruby 2024