Added backfill entry

sql/moz-fx-data-shared-prod/firefox_desktop_background_defaultagent_derived/default_agent_agg_v1/backfill.yaml

@@ -1,3 +1,13 @@
+2025-01-27:
+  start_date: 2023-01-01
+  end_date: 2023-12-31
+  reason: https://mozilla-hub.atlassian.net/browse/DENG-6889
+  watchers:
+  - wichan@mozilla.com
+  status: Initiate
+  shredder_mitigation: false
+  override_retention_limit: false
+
 2024-12-27:
   start_date: 2023-01-01
   end_date: 2023-12-31