feat(handlers): attempt to cancel ongoing imports during graceful shutdown

mentioned in commit 8c16a662

mentioned in issue #715 (closed)

mentioned in merge request gitlab-org/build/CNG!1060 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1891 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1892 (merged)

mentioned in merge request omnibus-gitlab!6181 (merged)

mentioned in merge request gitlab-development-kit!2656 (merged)