Add a note about deprecating /register with a user property. (#15703)

Application services providing a "user" property (instead of "username") for
the /register endpoint was never specified. Deprecate this very old
fallback.

changelog.d/15703.removal

@@ -0,0 +1 @@
+Deprecate calling the `/register` endpoint with an unspecced `user` property for application services.

docs/upgrade.md

@@ -88,6 +88,17 @@ process, for example:
     dpkg -i matrix-synapse-py3_1.3.0+stretch1_amd64.deb
     ```
 
+# Upgrading to v1.85.0
+
+## Application service registration with "user" property deprecation
+
+Application services should ensure they call the `/register` endpoint with a
+`username` property. The legacy `user` property is considered deprecated and
+should no longer be included.
+
+A future version of Synapse (v1.88.0 or later) will remove support for legacy
+application service login.
+
 # Upgrading to v1.84.0
 
 ## Deprecation of `worker_replication_*` configuration settings