Update UPGRADE-6.0.md

Co-authored-by: Copilot <[email protected]>

Author: bbrala

UPGRADE-6.0.md

@@ -30,6 +30,8 @@ Thank you for your support and contributions to the project.
 
 * The signature for the `BaseConstraint::AddError` method has changed.
 
+  The `$message` parameter has been removed and replaced by the `ConstraintError` parameter. 
+  The `ConstraintError` object encapsulates the error message along with additional information about the constraint violation.
 
     *Before*
     ```php