Retire puppet-monasca

This module still depends on outdated interface of puppet-python which
was already removed. Because we haven't seen any intention to keep it
maintained[1], I'd propose retiring the module now.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2021-August/024118.html

Depends-on: https://review.opendev.org/805103/
Change-Id: Ib99cda66f99caeb1e6affd198c589de1124ccd29

reference/legacy.yaml

@@ -1808,6 +1808,10 @@ Puppet OpenStack:
       repos:
         - openstack/puppet-glare
       retired-on: 2021-05-11
+    puppet-monasca:
+      repos:
+        - openstack/puppet-monasca
+      retired-on: 2021-08-19
     puppet-openstack-specs:
       release-management: none
       repos:

reference/projects.yaml

@@ -2435,9 +2435,6 @@ Puppet OpenStack:
     puppet-mistral:
       repos:
         - openstack/puppet-mistral
-    puppet-monasca:
-      repos:
-        - openstack/puppet-monasca
     puppet-murano:
       repos:
         - openstack/puppet-murano