feat(handlers): log warning when eligibility flag is not set in migration mode

mentioned in commit fe807a4a

mentioned in merge request gitlab-org/build/CNG!746 (merged)

mentioned in commit e6743bfb

mentioned in merge request omnibus-gitlab!5562 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1155 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1156 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2156 (merged)

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