Flag ActiveSupport::JSON in Gitlab::Json Cop

.rubocop.yml

...migrate/20190517153211_migrate_k8s_service_integration.rb

ee/spec/lib/gitlab/geo/oauth/session_spec.rb

lib/gitlab/import_export/json/legacy_reader.rb

lib/gitlab/import_export/json/ndjson_reader.rb

lib/gitlab/import_export/lfs_restorer.rb

lib/gitlab/json_cache.rb

rubocop/cop/gitlab/json.rb

spec/features/projects/import_export/export_file_spec.rb

...b/gitlab/import_export/group/legacy_tree_restorer_spec.rb

spec/lib/gitlab/import_export/group/tree_restorer_spec.rb

spec/lib/gitlab/import_export/import_test_coverage_spec.rb

spec/lib/gitlab/json_cache_spec.rb

spec/requests/git_http_spec.rb

spec/rubocop/cop/gitlab/json_spec.rb

spec/support/import_export/common_util.rb