Add upgrade note about property hooks (#11636)

People that might have experimented with property hooks while still
using ORM < 2.20.0 need to know that they need to remove their
experiment or upgrade to a version that explicitly supports them.

UPGRADE.md

@@ -1,5 +1,13 @@
 # Upgrade to 2.20
 
+## Explictly forbid property hooks
+
+Property hooks are not supported yet by Doctrine ORM. Until support is added,
+they are explicitly forbidden because the support would result in a breaking
+change in behavior.
+
+Progress on this is tracked at https://github.com/doctrine/orm/issues/11624 .
+
 ## PARTIAL DQL syntax is undeprecated for non-object hydration
 
 Use of the PARTIAL keyword is not deprecated anymore in DQL when used with a hydrator