Add backfill incident escalation status migration

lib/gitlab/background_migration/backfill_incident_issue_escalation_statuses.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # BackfillIncidentIssueEscalationStatuses adds
+    # IncidentManagement::IssuableEscalationStatus records for existing Incident issues.
+    # They will be added with no policy, and escalations_started_at as nil.
+    class BackfillIncidentIssueEscalationStatuses
+      def perform(start_id, stop_id)
+        ActiveRecord::Base.connection.execute <<~SQL
+          INSERT INTO incident_management_issuable_escalation_statuses (issue_id, created_at, updated_at)
+            SELECT issues.id, now(), now()
+            FROM issues
+            WHERE issues.issue_type = 1
+            AND issues.id BETWEEN #{start_id} AND #{stop_id}
+            ON CONFLICT (issue_id) DO NOTHING;
+        SQL
+      end
+    end
+  end
+end