Add feature flag toggle for elastic migration worker

Changelog: changed
EE: true

config/feature_flags/ops/elastic_migration_worker.yml

+---
+name: elastic_migration_worker
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80310
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/352424
+milestone: '14.8'
+type: ops
+group: group::global search
+default_enabled: true

ee/app/workers/elastic/migration_worker.rb

@@ -17,6 +17,7 @@ module Elastic
     urgency :throttled
 
     def perform
+      return false if Feature.disabled?(:elastic_migration_worker, type: :ops, default_enabled: :yaml)
       return false unless Gitlab::CurrentSettings.elasticsearch_indexing?
       return false unless helper.alias_exists?
 

ee/spec/workers/elastic/migration_worker_spec.rb

@@ -6,6 +6,18 @@ RSpec.describe Elastic::MigrationWorker, :elastic do
   subject { described_class.new }
 
   describe '#perform' do
+    context 'Feature Flag `elastic_migration_worker` is disabled' do
+      before do
+        stub_feature_flags(elastic_migration_worker: false)
+        stub_ee_application_setting(elasticsearch_indexing: true)
+      end
+
+      it 'returns with no execution' do
+        expect(subject).not_to receive(:execute_migration)
+        expect(subject.perform).to be_falsey
+      end
+    end
+
     context 'indexing is disabled' do
       before do
         stub_ee_application_setting(elasticsearch_indexing: false)