update changeLog

Author: harshithad0703

CHANGELOG.md

@@ -2,6 +2,7 @@
 ## [v1.21.2](https://github.com/contentstack/contentstack-management-javascript/tree/v1.21.2) (2025-05-19)
  - Enhancement
    - Added preview token generation.
+   - Update addSettings Method to Support Generic Stack Settings Update
 
 ## [v1.21.1](https://github.com/contentstack/contentstack-management-javascript/tree/v1.21.1) (2025-05-12)
  - Fix