Announce Python / PostgreSQL deprecation policies (#9085)

Fixes #8782

CHANGES.md

@@ -1,3 +1,31 @@
+Synapse 1.25.0 (2021-01-13)
+==============================
+
+Ending Support for Python 3.5 and Postgres 9.5
+----------------------------------------------
+
+With this release, the Synapse team is announcing a formal deprecation policy for our platform dependencies, like Python and PostgreSQL:
+
+All future releases of Synapse will follow the upstream end-of-life schedules.
+
+Which means:
+
+* This is the last release which guarantees support for Python 3.5.
+* We will end support for PostgreSQL 9.5 early next month.
+* We will end support for Python 3.6 and PostgreSQL 9.6 near the end of the year.
+
+Crucially, this means __we will not produce .deb packages for Debian 9 (Stretch) or Ubuntu 16.04 (Xenial)__ beyond the transition period described below.
+
+The website https://endoflife.date/ has convenient summaries of the support schedules for projects like [Python](https://endoflife.date/python) and [PostgreSQL](https://endoflife.date/postgresql).
+
+If you are unable to upgrade your environment to a supported version of Python or Postgres, we encourage you to consider using the [Synapse Docker images](../INSTALL.md#docker-images-and-ansible-playbooks) instead.
+
+### Transition Period
+
+We will make a good faith attempt to avoid breaking compatibility in all releases through the end of March 2021. However, critical security vulnerabilities in dependencies or other unanticipated circumstances may arise which necessitate breaking compatibility earlier.
+
+We intend to continue producing .deb packages for Debian 9 (Stretch) and Ubuntu 16.04 (Xenial) through the transition period.
+
 Synapse 1.25.0rc1 (2021-01-06)
 ==============================
 

UPGRADE.rst

@@ -5,6 +5,16 @@ Before upgrading check if any special steps are required to upgrade from the
 version you currently have installed to the current version of Synapse. The extra
 instructions that may be required are listed later in this document.
 
+* Check that your versions of Python and PostgreSQL are still supported.
+
+  Synapse follows upstream lifecycles for `Python`_ and `PostgreSQL`_, and
+  removes support for versions which are no longer maintained.
+
+  The website https://endoflife.date also offers convenient summaries.
+
+  .. _Python: https://devguide.python.org/devcycle/#end-of-life-branches
+  .. _PostgreSQL: https://www.postgresql.org/support/versioning/
+
 * If Synapse was installed using `prebuilt packages
   <INSTALL.md#prebuilt-packages>`_, you will need to follow the normal process
   for upgrading those packages.
@@ -78,6 +88,18 @@ for example:
 Upgrading to v1.25.0
 ====================
 
+Last release supporting Python 3.5
+----------------------------------
+
+This is the last release of Synapse which guarantees support with Python 3.5,
+which passed its upstream End of Life date several months ago.
+
+We will attempt to maintain support through March 2021, but without guarantees.
+
+In the future, Synapse will follow upstream schedules for ending support of
+older versions of Python and PostgreSQL. Please upgrade to at least Python 3.6
+and PostgreSQL 9.6 as soon as possible.
+
 Blacklisting IP ranges
 ----------------------