Add CF-related dates for Baltic Ruby 2026

Author: okuramasafumi

_data/conferences.yml

@@ -3317,6 +3317,9 @@
   twitter: balticruby
   mastodon: https://ruby.social/@balticruby
   announced_on: 2025-09-02
+  cfp_open_date: 2025-09-02
+  cfp_close_date: 2025-12-31
+  cfp_link: "https://www.papercall.io/balticruby2026"
 
 - name: Rails World 2026
   location: Austin, TX