Fix supported releases and py versions in the doc

Our documentation showed that Victoria, Ussuri and Train are the
supported releases by tempest's master, which wasn't the case.
It also showed that tempest is compatible with python 3.6, 3.7
and 3.8. We dropped py 3.6 and 3.7 support by [1].

[1] https://review.opendev.org/c/openstack/tempest/+/843182

Closes-Bug: #1976282
Change-Id: I0ccfa824f08cbdf58f26a142dac3a0881605ef08
diff --git a/doc/source/supported_version.rst b/doc/source/supported_version.rst
index 4ca7f0d..f630578 100644
--- a/doc/source/supported_version.rst
+++ b/doc/source/supported_version.rst
@@ -9,9 +9,10 @@
 
 Tempest master supports the below OpenStack Releases:
 
+* Yoga
+* Xena
+* Wallaby
 * Victoria
-* Ussuri
-* Train
 
 For older OpenStack Release:
 
@@ -32,6 +33,5 @@
 
 Tempest master supports the below python versions:
 
-* Python 3.6
-* Python 3.7
 * Python 3.8
+* Python 3.9