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
1 file changed