fix(datastore): importer: consistently store layer blob media types in database

mentioned in commit 56e56b52

mentioned in issue #999 (closed)

mentioned in merge request gitlab-org/build/CNG!1384 (merged)

mentioned in merge request gitlab-org/charts/gitlab!3145 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2708 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2714 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2715 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2737 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2738 (merged)

mentioned in merge request gitlab-development-kit!3152 (merged)