Disable updating the historical vulnerability statistics by default

Historical vulnerability statistics supposed to be updated once a day
by a cronjob because updating them always for each update/insert of a
vulnerability can reduce the throughput of our worker classes and API
endpoints.

With this change, to be able to observe the side effects of disabling
the update logic of historical vulnerability statistics, we are moving
it behind a feature flag. We will remove the logic and the feature flag
eventually.

Changelog: other
EE: true