Update Release Notes

Signed-off-by: camille-bouvy-frequenz <camille.bouvy@frequenz.com>

RELEASE_NOTES.md

@@ -6,7 +6,9 @@
 
 ## Upgrading
 
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+- Refactor order states:
+  - Add new order state PARTIALLY_FILLED_AND_CANCELED
+  - Remove obsolete order states CANCEL_REQUESTED and CANCEL_REJECTED
 
 ## New Features