Bump cirros version to 0.6.1

Cirros has made a fresh release, let us use it. Switch the download URLs
to https and drop an old example that no longer is available.

Depends-On: https://review.opendev.org/c/openstack/tempest/+/871271
Change-Id: I1d391b871fc9bfa825db30db9434922226b94d8a
diff --git a/doc/source/guides/nova.rst b/doc/source/guides/nova.rst
index 5b42797..d0fb274 100644
--- a/doc/source/guides/nova.rst
+++ b/doc/source/guides/nova.rst
@@ -122,7 +122,7 @@
 .. code-block:: shell
 
   $ openstack --os-compute-api-version 2.37 server create --flavor cirros256 \
-      --image cirros-0.3.5-x86_64-disk --nic none --wait test-server
+      --image cirros-0.6.1-x86_64-disk --nic none --wait test-server
 
 .. note:: ``--os-compute-api-version`` greater than or equal to 2.37 is
           required to use ``--nic=none``.