Add CfP details for RubyConf Thailand 2026 (#1074)

We opened the CfP yesterday and it will stay up until end of June.

Author: citizen428

_data/conferences.yml

@@ -3137,6 +3137,9 @@
   url: https://rubyconfth.com
   twitter: rubyconfth
   announced_on: 2025-02-28
+  cfp_open_date: 2025-05-26
+  cfp_close_date: 2025-06-30
+  cfp_link: https://www.papercall.io/rubyconfth2026
 
 - name: Tropical on Rails 2026
   location: São Paulo, Brazil