Add some missing implemented microversion in doc

InstanceActionsV221TestJSON tests v2.21 and
LiveMigrationRemoteConsolesV26Test tests v2.6, so
this is to add v2.21 and v2.6 in microversion_testing doc.

Change-Id: I3a8d76e484e10a10a2a53f5a9b084123eeb7b0d3

doc/source/microversion_testing.rst

@@ -302,6 +302,10 @@ Microversion tests implemented in Tempest
 
   .. _2.2: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id2
 
+  * `2.6`_
+
+  .. _2.6: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id5
+
   * `2.10`_
 
   .. _2.10: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id9
@@ -310,6 +314,10 @@ Microversion tests implemented in Tempest
 
   .. _2.20: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id18
 
+  * `2.21`_
+
+  .. _2.21: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id19
+
   * `2.25`_
 
   .. _2.25: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#maximum-in-mitaka