ci: add alert manager version constraint to match production

renovate-versions-gitlab-com.json

@@ -29,6 +29,13 @@
       ],
       "allowedVersions": "< 2.35.0"
     },
+    {
+      "matchPackageNames": [
+        "prometheus/alertmanager",
+        "description: alertmanager should match production version: https://alerts.gitlab.net/#/status"
+      ],
+      "allowedVersions": "< 0.23.0"
+    },
     {
       "matchPackageNames": [
         "python/cpython",