Update notes about CentOS support

Remove notes referring to old releases (Train, Victoria). Add a note to
cover migration to RL9.

Change-Id: I57bcc9c3967fb6cdea56cb9a252255322ec2f1c9

doc/source/user/support-matrix.rst

@@ -9,17 +9,9 @@ Kolla Ansible supports the following host Operating Systems (OS):
 
 .. note::
 
-   CentOS 7 is no longer supported as a host OS. The Train release supports
-   both CentOS 7 and 8, and provides a route for migration. See the `Kolla
-   Ansible Train documentation
-   <https://docs.openstack.org/kolla-ansible/train/user/centos8.html>`_ for
-   information on migrating to CentOS 8.
-
-.. note::
-
-   CentOS Linux 8 (as opposed to CentOS Stream 8) is no longer supported as a
-   host OS. The Victoria release will in future support both CentOS Linux 8 and
-   CentOS Stream 8, and provides a route for migration.
+   CentOS Stream 8 is no longer supported as a host OS. The Yoga release
+   supports both CentOS Stream 8 and CentOS Stream 9 / Rocky Linux 9, and
+   provides a route for migration.
 
 .. note::
 
@@ -27,8 +19,6 @@ Kolla Ansible supports the following host Operating Systems (OS):
    based images. Users can build them on their own. We recommend using Rocky
    Linux 9 images instead.
 
-
-
 * CentOS Stream 9
 * Debian Bullseye (11)
 * openEuler 22.03 LTS