Ignore `start_date_sourcing_milestone_id` col on `vulnerabilities`

What does this MR do and why?

start_date_sourcing_milestone_id is always nil in production:

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

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

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

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Changelog: deprecated

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Related to #268154 (closed)