C#: Add change note

change-notes/1.21/analysis-csharp.md

@@ -6,6 +6,7 @@
 |------------------------------|------------------------|-----------------------------------|
 | Class defines a field that uses an ICryptoTransform class in a way that would be unsafe for concurrent threads (`cs/thread-unsafe-icryptotransform-field-in-class`) | Fewer false positive results | The criteria for a result has changed to include nested properties, nested fields and collections. The format of the alert message has changed to highlight the static field. |
 | Constant condition (`cs/constant-condition`) | Fewer false positive results | Results have been removed where the `null` value is in a conditional expression on the left hand side of a null-coalescing expression. For example, in `(a ? b : null) ?? c`, `null` is not considered to be a constant condition. |
+| Useless upcast (`cs/useless-upcast`) | Fewer false positive results | Results have been removed where the upcast is used to disambiguate the target of a constructor call. |
 
 ## Changes to code extraction