Announce Python 3.12 migration

Author: xhochy

src/user/announcements.rst

@@ -8,6 +8,21 @@ Our announcements are published to an RSS feed `here <https://conda-forge.org/do
 2023
 ----
 
+2023-09-25: Python 3.12 migration and Python 3.11 by default
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+With the Python 3.12 release approaching, we have already started the rebuild of packages for it.
+Although, there is no offical Python 3.12 release yet, the release candidates of it will have the same ABI.
+Thus packages built with the release candidate can be safely used with the later offical release.
+To support rebuilding packages on conda-forge while ensuring Python release candidates don't end up in end-user solves, we have uploaded the Python 3.12.0rc2 and rc3 builds to the ``conda-forge/label/python_rc`` channel.
+The ``python312`` migration adds this channel in the feedstock builds to the Python 3.12 matrix entry.
+On the offical release of Python 3.12, we will adjust the migration and remove the channel again.
+Then (on a rerender), feedstock will only consume the main channel again.
+
+Overall, this approach allows us to provide Python 3.12 for a wide range of packages already on the day of the offical Python 3.12 release.
+
+At the same time, we have stopped the Python 3.11 migration and added it to the list of default Python versions on conda-forge.
+
 2023-08-24: Bumping Minimum MacOS version to 10.13
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^