Extend CFP closing date for RubyConf Africa (#651)

Author: banta

_data/conferences.yml

@@ -2634,7 +2634,7 @@
   url: https://rubyconf.africa/
   twitter: ruby_african
   cfp_open_date: 2024-03-20
-  cfp_close_date: 2024-05-31
+  cfp_close_date: 2024-07-20
   cfp_link: https://www.papercall.io/rubyconfafrica2024
 
 - name: Madison+ Ruby 2024