Retire PowerVMStacker SIG

The PowerVMStacker SIG has been inactive for ~5 years so the SIG and
its repositories should be retired. See [1] for further details.

[1] https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/thread/S2GYZGXYGFMHMQ5CWF5P52BRW2YU4TVV/

Depends-on: https://review.opendev.org/c/openstack/nova-powervm/+/909536
Depends-on: https://review.opendev.org/c/openstack/ceilometer-powervm/+/909537
Depends-on: https://review.opendev.org/c/openstack/networking-powervm/+/909538
Change-Id: Ia5303c35af6b4e08880ee3b4268506d2f4aec0c5

reference/legacy.yaml

@@ -1835,6 +1835,20 @@ PowerVMStackers:
       tags:
         - stable:follows-policy
 
+PowerVMStackers SIG:
+  irc-channel: openstack-powervm
+  retired-on: 2024-02-20
+  deliverables:
+    ceilometer-powervm:
+      repos:
+        - openstack/ceilometer-powervm
+    networking-powervm:
+      repos:
+        - openstack/networking-powervm
+    nova-powervm:
+      repos:
+        - openstack/nova-powervm
+
 Puppet OpenStack:
   partial: yes
   deliverables:

reference/sigs-repos.yaml

@@ -58,10 +58,6 @@ Packaging:
   - repo: openstack/rpm-packaging-tools
   - repo: openstack/pymod2pkg
   - repo: openstack/renderspec
-PowerVMStackers:
-  - repo: openstack/ceilometer-powervm
-  - repo: openstack/networking-powervm
-  - repo: openstack/nova-powervm
 Public Cloud SIG:
   - repo: openstack/publiccloud-sig
 Security: