compute: Add missing 2.73 response schema validation

Microversion v2.73 was introduced in Nova by I46edd595e7417c58410648
changing the request responses of ``GET servers/{server_id}``, ``GET
/servers/detail``, ``POST /servers/{server_id}/action``  where the
action is rebuild and ``PUT servers/{server_id}``.

Change-Id: I8c800f6e280ec6fdf204d344124d994a25d0af19
diff --git a/doc/source/microversion_testing.rst b/doc/source/microversion_testing.rst
index b4f06e3..50fd4f2 100644
--- a/doc/source/microversion_testing.rst
+++ b/doc/source/microversion_testing.rst
@@ -414,6 +414,10 @@
 
   .. _2.71: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id64
 
+  * `2.73`_
+
+  .. _2.73: https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id66
+
 * Volume
 
   * `3.3`_