[monasca-tempest-plugin] Tag xena-last

Xena branch is in Extended Maintenance now[1]. Tempest and
tempest plugins are branchless which means master version of
Tempest and its plugins is used to test the supported stable
branches.

Once any stable branch is moved to EM state then, Tempest and
its plugins compatible tag[2] needs to be released so that we can
keep testing the EM stable branches with that tag once master
Tempest and its plugins are not compatible[3].

Tagging 'xena-last' with the latest hash which is nothing but 2.6.0
tag.

[1] https://review.opendev.org/c/openstack/releases/+/881254
[2] https://docs.openstack.org/tempest/latest/stable_branch_support_policy.html
[3] https://docs.openstack.org/tempest/latest/tempest_and_plugins_compatible_version_policy.html

Change-Id: Ic0adf06fd4115f19f00652f2785905ccb828fad5

deliverables/xena/monasca-tempest-plugin.yaml

@@ -13,3 +13,7 @@ releases:
     projects:
       - repo: openstack/monasca-tempest-plugin
         hash: 062cb7576a574d5fd12f75bea351e6381b19a5a1
+  - version: xena-last
+    projects:
+      - repo: openstack/monasca-tempest-plugin
+        hash: 4163264c551318e96ce11cc0cadb5c78de5f8bf8