Add notes about dropping support for Python 3.6 and Postgres 9.6.

CHANGES.md

@@ -3,6 +3,10 @@ Synapse 1.49.1 (2021-12-21)
 This release fixes a regression introduced in Synapse 1.49.0 which could cause `/sync` requests to take significantly longer. This would particularly affect "initial" syncs for users participating in a large number of rooms, and in extreme cases, could make it impossible for such users to log in on a new client.
+**Note:** in line with our [deprecation policy](https://matrix-org.github.io/synapse/latest/deprecation_policy.html) for platform dependencies, this will be the last release to support Python 3.6 and PostgreSQL 9.6, both of which have now reached upstream end-of-life. Synapse will require Python 3.7+ and PostgreSQL 10.
+
+**Note:** We will also stop producing packages for Ubuntu 18.04 (Bionic Beaver) after this release, as it uses Python 3.6.
+
 Bugfixes
 --------

docs/deprecation_policy.md

@@ -14,8 +14,8 @@ i.e. when a version reaches End of Life Synapse will withdraw support for that
 version in future releases.
 
 Details on the upstream support life cycles for Python and PostgreSQL are
-documented at https://endoflife.date/python and
-https://endoflife.date/postgresql.
+documented at [https://endoflife.date/python](https://endoflife.date/python) and
+[https://endoflife.date/postgresql](https://endoflife.date/postgresql).
 
 
 Context