docs: add reno and upgrade info for CentOS Stream

Story: 2008795
Task: 42459

Change-Id: I1f55a2f47ac614fea983e19802e469efd810e849

doc/source/upgrading.rst

@@ -6,6 +6,15 @@ Upgrading
 
 This section describes how to upgrade from one OpenStack release to another.
 
+CentOS Stream 8
+---------------
+
+The Wallaby release adds support for CentOS Stream 8 as a host operating
+system. CentOS Stream 8 support will also be added to a Victoria stable
+release. CentOS Linux users upgrading from Victoria should first migrate hosts
+and container images from CentOS Linux to CentOS Stream before upgrading to
+Wallaby.
+
 Preparation
 ===========
 

releasenotes/notes/centos-stream-cb683a092153b93e.yaml

+---
+features:
+  - |
+    Adds support for CentOS Stream 8 as a host Operating System and base
+    container image. This is the only distribution of CentOS supported from
+    the Wallaby release. The Victoria release will support both CentOS Linux 8
+    and CentOS Stream 8 hosts and images, and provides a route for migration.
+upgrade:
+  - |
+    CentOS Linux 8 is no longer supported as a host Operating System or base
+    container image. CentOS users should migrate to CentOS Stream 8. The
+    Victoria release will support both CentOS Linux 8 and CentOS Stream 8
+    hosts and images, and provides a route for migration.