Add changelog for v2.20

Changelog.md

 # Changelog
 
+## [2.20] - 2024-10-10
+-  Fix callback step live update by bumping to orchestrator-core==2.7.6
+
 ## [2.19] - 2024-10-09
 - LSO interaction rework: Skip a playbook automatically if the inventory is empty.
 - Introduction of LSOState type.