Move postgres warning banner to top of readme

Author: David Robertson

CHANGES.md

@@ -1,6 +1,8 @@
 Synapse 1.72.0 (2022-11-22)
 ===========================
 
+Please note that we now only support PostgreSQL 11+, because PostgreSQL 10 has reached end-of-life, c.f. our [Deprecation Policy](https://github.com/matrix-org/synapse/blob/develop/docs/deprecation_policy.md).
+
 Bugfixes
 --------
 
@@ -10,8 +12,6 @@ Bugfixes
 Synapse 1.72.0rc1 (2022-11-16)
 ==============================
 
-Please note that we now only support PostgreSQL 11+, because PostgreSQL 10 has reached end-of-life, c.f. our [Deprecation Policy](https://github.com/matrix-org/synapse/blob/develop/docs/deprecation_policy.md).
-
 Features
 --------