fix(gc/worker): delete task if dangling manifest no longer exists on database

mentioned in commit 6d4f158b

mentioned in issue #458 (closed)

mentioned in merge request gitlab-org/build/CNG!767 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1254 (merged)

mentioned in merge request omnibus-gitlab!5623 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1258 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1259 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2194 (merged)

mentioned in merge request gitlab-development-kit!2385 (merged)