This implements vulnerability counts on the `SecurityReportsApp`
component, implemented behind a disabled-by-default feature flag
`core_security_mr_widget_counts`, as part of
https://gitlab.com/gitlab-org/gitlab/-/issues/273423.

This cannot be enabled until the backend endpoints are modified to be
usable in non-Ultimate plans. See
https://gitlab.com/gitlab-org/gitlab/-/issues/284689 for more details.