fix: add tracking parameter to closed conference link

Author: Copilot

_data/closed.yml

@@ -2,5 +2,5 @@
   location: Zurich, Switzerland
   first_date: June 2, 2023
   last_date: May 23, 2025
-  url: https://www.greatestquality.ch
+  url: https://www.greatestquality.ch?utm_source=testingconferences
   status: Annual conference discontinued after 2025 and replaced by smaller community meetups.