Make vulnerability counts available for Free users

This issue tracks the frontend work necessary to bring vulnerability counts in the security MR widget to Free.

In #273423 (closed) we wanted to make vulnerability counts available for all non-Ultimate users. It turned out that backend work was necessary to make that happen (i.e., CE/FOSS). That work is implemented behind a feature flag.

It may be that this issue can be closed in favour of #284097 (closed); it's not yet clear how much - if any - frontend work is still required once the backend is in place.