write background migrations for updating confidence levels `undefined` to `unknown`

Problem to solve

We no longer report undefined confidence level for vulnerabilities however we need to create background migration for the ones that are already in db

Proposal

Write background migrations for updating confidence levels undefined to unknown . Migrations will be very similar to work done in #198158 (closed)

update the rails backend to remove the Undefined value and migrate existing records in DB having Undefined to Unknown

Implementation plan

Migrate Undefined to Unknown values in db for vulnerabilities and vulnerability_occurrences table