Change schema hypervisor.cpu_info from string to object in 2.28

This is to change schema hypervisor.cpu_info from string to object
in microversion 2.28.
https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id25

Change-Id: I36715c6b1751ceee304d18114b4473d60758a7ff
diff --git a/doc/source/microversion_testing.rst b/doc/source/microversion_testing.rst
index ce9bbb5..2059650 100644
--- a/doc/source/microversion_testing.rst
+++ b/doc/source/microversion_testing.rst
@@ -334,6 +334,10 @@
 
   .. _2.26: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id23
 
+* `2.28`_

+

+  .. _2.28: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id25
+
   * `2.32`_
 
   .. _2.32: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id29