Mention dropped Postgres v9.6 support in the changelog

Related to c4d2c8394c

CHANGELOG.md

@@ -1,3 +1,12 @@
+# 2021-11-11
+
+## Dropped support for Postgres v9.6
+
+Postgres v9.6 reached its end of life today, so the playbook will refuse to run for you if you're still on that version.
+
+Synapse still supports v9.6 (for now), but we're retiring support for it early, to avoid having to maintain support for so many Postgres versions. Users that are still on Postgres v9.6 can easily [upgrade Postgres](docs/maintenance-postgres.md#upgrading-postgresql) via the playbook.
+
+
 # 2021-10-23
 
 ## Hangouts bridge no longer updated, superseded by a Googlechat bridge