fix(configuration): require rootdirectory to be set when in migration mode

mentioned in commit 515e422e

mentioned in issue #443 (closed)

mentioned in merge request gitlab-org/build/CNG!753 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2172 (merged)

mentioned in merge request omnibus-gitlab!5585 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1215 (merged)

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