Properly switch the "2.X.Y" image to point to "newest" Python

The change #36003 attempted to change which Python version is used
for the "default" image ("2.8.0" and effectively "latest as well).

But it failed to do so - because of a bug. The changelog about this
have been removed in #36777 and we are bringing the change back
(this time properly implemented) and changelog - with aim to
release it in Airflow 2.9.0.

Fixes: #36740

dev/breeze/src/airflow_breeze/commands/release_management_commands.py

@@ -1536,7 +1536,7 @@ def release_prod_images(
                 "--push",
             ]
             run_command(docker_buildx_command)
-            if python == DEFAULT_PYTHON_MAJOR_MINOR_VERSION:
+            if python == ALLOWED_PYTHON_MAJOR_MINOR_VERSIONS[-1]:
                 alias_image(
                     slim_image_name,
                     f"{dockerhub_repo}:slim-{airflow_version}",
@@ -1568,7 +1568,7 @@ def release_prod_images(
                 "--push",
             ]
             run_command(docker_buildx_command)
-            if python == DEFAULT_PYTHON_MAJOR_MINOR_VERSION:
+            if python == ALLOWED_PYTHON_MAJOR_MINOR_VERSIONS[-1]:
                 alias_image(image_name, f"{dockerhub_repo}:{airflow_version}")
     # in case of re-tagging the images might need few seconds to refresh multi-platform images in DockerHub
     time.sleep(10)

docs/docker-stack/changelog.rst

@@ -34,6 +34,18 @@ the Airflow team.
        any Airflow version from the ``Airflow 2`` line. There is no guarantee that it will work, but if it does,
        then you can use latest features from that image to build images for previous Airflow versions.
 
+Airflow 2.9
+~~~~~~~~~~~
+
+   * The "latest" image (i.e. default Airflow image when ``apache/airflow`` is used or
+     ``apache/airflow:slim-latest``) uses now the newest supported Python version. Previously it was using
+     the "default" Python version which was Python 3.8 as of Airflow 2.8. With Airflow reference images
+     released for Airflow 2.9.0, the images are going to use Python 3.12 as this is the latest supported
+     version for Airflow 2.9 line. Users can use Python 3.8 by using ``apache/airflow:2.9.0-python3.8`` and
+     ``apache/airflow:slim-2.9.0-python-3.8`` images respectively so while the change is potentially
+     breaking, it is very easy to switch to the previous behaviour.
+
+
 Airflow 2.8
 ~~~~~~~~~~~