Update Yoga testing runtime

As discussed in Yoga PTG[1] and centos stream 9
is released, we are updating Yoga testing runtime with:
- Add Debian 11 as tested distro
- Change centos stream 8 -> centos stream 9
- Bump lowest python version to test to 3.8
  and highest to python 3.9

[1] https://etherpad.opendev.org/p/tc-yoga-ptg

Change-Id: I2fb3a1daa5ee812ea45bab0c2fb7c0997dc2d116

reference/project-testing-interface.rst

@@ -116,7 +116,8 @@ The following free operating systems are representative of platforms regularly
 used to deploy OpenStack on:
 
 - `Latest Ubuntu LTS <https://wiki.ubuntu.com/Releases>`_
-- `Latest CentOS Major <https://www.centos.org/download/>`_
+- `Latest CentOS Stream Major <https://www.centos.org/download/>`_
+- `Latest Debian Stable <https://www.debian.org/releases/>`_
 
 Below are the two upstream testing required as minimum to consider the above distro for defining
 the testing runtime:

reference/runtimes/yoga.rst

@@ -6,7 +6,8 @@ At the start of the Yoga development cycle, the current :ref:`LTS or stable
 distribution <pti-linux-distros>` versions are:
 
 * Ubuntu 20.04
-* CentOS 8 Stream
+* CentOS Stream 9
+* Debian 11
 
 Python Runtimes for Yoga
 ========================
@@ -20,8 +21,8 @@ when all projects have migrated to a later version.
 Based on the criteria above, all Python-based projects must target and test
 against, at a minimum:
 
-* Python 3.6 (default in CentOS 8 Stream)
+* Python 3.8 (available as default in Ubuntu 20.04)
-* Python 3.8 (latest available; default in Ubuntu 20.04)
+* Python 3.9 (available as default in Debian 11)
 
 More details on Python requirements can be found in :ref:`pti-python`.