Retire devstack-gate

This repo was deprecated in xena after devstack grew the ability to self
bootstrap CI environments using ansible. We want to drop this repo as it
is no longer necessary and it will help clear the path for Ubuntu Xenial
cleanup in the CI system.

Depends-On: https://review.opendev.org/c/openstack/devstack-gate/+/919626
Change-Id: I232939fc9c7ba13aea85c9c179d4517eb4ba33d5

reference/legacy.yaml

@@ -2174,6 +2174,14 @@ Telemetry:
       repos:
         - openstack/python-pankoclient
 
+Testing and Collaboration Tools:
+  partial: yes
+  deliverables:
+    devstack-gate:
+      retired-on: 2024-05-14
+      repos:
+        - openstack/devstack-gate
+
 tripleo:
   irc-channel: tripleo
   service: Deployment service

reference/sigs-repos.yaml

@@ -78,8 +78,6 @@ Security:
 Testing and Collaboration Tools:
   - repo: openstack/bugdaystats
   - repo: openstack/ci-log-processing
-  - repo: openstack/devstack-gate
-    deprecated: xena
   - repo: openstack/diskimage-builder
   - repo: openstack/logstash-filters
   - repo: openstack/nose-html-output