TEIS-2027: added changelog entry for possible breaking change about...

TEIS-2027: added changelog entry for possible breaking change about replacement of Gradle compile -> implementation

CHANGELOG.md

 # Changelog
 
+## [1.21.0] -
+
+* NB: replaced Gradle compile with implementation which doesn't expose service libraries to depending services. Services
+  which depend on latest versions of office-api-gateway-lib may require to add dependencies for Gradle task
+  integTestImplementation in service Gradle configuration
+
 ## [1.20.0] - 2021-02-22
 
 * moved the preview and generate functionality to test controller