Ignore `start_date_sourcing_milestone_id` col on `vulnerabilities`

`start_date_sourcing_milestone_id` is always `nil` in production:

```sh
[ gstg ] production> Vulnerability.where.not(start_date_sourcing_milestone_id: nil).count
=> 0
[ gstg ] production>
```

It was added [when the initial vulnerability table][1] was created, however
appears to have never been used.

This MR ignores the column for step 1 of the
[3-MR drop column process][0]

related to: #268154
Changelog: deprecated

[0]:https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-columns
[1]:8ad1881c