Finalize BBM for vulnerability risk score on self-managed (Security Dashboard GA Release)

Finalize the BBM introduced with #569091 (closed) in %18.6 after the required stop milestone in %18.9.

Note that finalization on gitlab.com and dedicated will be implemented in %18.7 itself tracked in #581522, we will have to no-op that migration and introduce a post deployment migration to finalize in %18.9 which can cover all installations including self-managed.