Group migration: Group labels are not migrated

Summary

When group migration is performed for group with labels, group labels are not migrated.

This might potentially be related to the way we test it by doing import of group from the same instance of gitlab

Steps to reproduce

Issue is reproducible by E2E test: qa/qa/specs/features/browser_ui/1_manage/group/bulk_import_group_spec.rb

What is the expected correct behavior?

Labels should be migrated

Relevant logs and/or screenshots

{"severity":"WARN","time":"2021-06-01T08:27:35.484Z","class":"BulkImports::ExportRequestWorker","args":["32"],"retry":25,"queue":"bulk_imports_export_request","backtrace":true,"version":0,"jid":"227e5dd8fdcd89594e12d6d8","created_at":"2021-06-01T08:24:47.880Z","meta.user":"qa-user-c2a82909a23d81a7","meta.caller_id":"BulkImportWorker","meta.remote_ip":"127.0.0.1","meta.feature_category":"importers","meta.client_id":"user/122","correlation_id":"1d9b0a6e11cdb2087a18f56209df15f1","enqueued_at":"2021-06-01T08:27:34.663Z","failed_at":"2021-06-01T08:25:01.652Z","retry_count":2,"retried_at":"2021-06-01T08:26:36.230Z","job_size_bytes":4,"pid":59774,"message":"BulkImports::ExportRequestWorker JID-227e5dd8fdcd89594e12d6d8: fail: 0.817557 sec","job_status":"fail","scheduling_latency_s":0.002168,"db_count":1,"db_write_count":0,"db_cached_count":0,"cpu_s":0.002287,"duration_s":0.817557,"completed_at":"2021-06-01T08:27:35.483Z","error_message":"Couldn't find BulkImports::Entity with 'id'=32","error_class":"ActiveRecord::RecordNotFound","error_backtrace":["app/workers/bulk_imports/export_request_worker.rb:14:in `perform'","lib/gitlab/sidekiq_middleware/duplicate_jobs/strategies/until_executing.rb:16:in `perform'","lib/gitlab/sidekiq_middleware/duplicate_jobs/duplicate_job.rb:40:in `perform'","lib/gitlab/sidekiq_middleware/duplicate_jobs/server.rb:8:in `call'","lib/gitlab/sidekiq_middleware/worker_context.rb:9:in `wrap_in_optional_context'","lib/gitlab/sidekiq_middleware/worker_context/server.rb:17:in `block in call'","lib/gitlab/application_context.rb:70:in `block in use'","lib/gitlab/application_context.rb:70:in `use'","lib/gitlab/application_context.rb:27:in `with_context'","lib/gitlab/sidekiq_middleware/worker_context/server.rb:15:in `call'","lib/gitlab/sidekiq_status/server_middleware.rb:7:in `call'","lib/gitlab/sidekiq_versioning/middleware.rb:9:in `call'","lib/gitlab/sidekiq_middleware/admin_mode/server.rb:14:in `call'","lib/gitlab/sidekiq_middleware/instrumentation_logger.rb:9:in `call'","lib/gitlab/sidekiq_middleware/batch_loader.rb:7:in `call'","lib/gitlab/sidekiq_middleware/extra_done_log_metadata.rb:7:in `call'","lib/gitlab/sidekiq_middleware/request_store_middleware.rb:10:in `block in call'","lib/gitlab/with_request_store.rb:17:in `enabling_request_store'","lib/gitlab/with_request_store.rb:10:in `with_request_store'","lib/gitlab/sidekiq_middleware/request_store_middleware.rb:9:in `call'","lib/gitlab/sidekiq_middleware/server_metrics.rb:25:in `block in call'","lib/gitlab/sidekiq_middleware/server_metrics.rb:48:in `block in instrument'","lib/gitlab/metrics/background_transaction.rb:30:in `run'","lib/gitlab/sidekiq_middleware/server_metrics.rb:48:in `instrument'","ee/lib/ee/gitlab/sidekiq_middleware/server_metrics.rb:17:in `instrument'","lib/gitlab/sidekiq_middleware/server_metrics.rb:24:in `call'","lib/gitlab/sidekiq_middleware/monitor.rb:8:in `block in call'","lib/gitlab/sidekiq_daemon/monitor.rb:49:in `within_job'","lib/gitlab/sidekiq_middleware/monitor.rb:7:in `call'","lib/gitlab/sidekiq_middleware/size_limiter/server.rb:13:in `call'","lib/gitlab/sidekiq_logging/structured_logger.rb:19:in `call'"],"db_duration_s":0.000316}