Ignore `milestone_id` column on `vulnerabilities` table

What does this MR do and why?

milestone_id is always nil in production:

[ gstg ] production> Vulnerability.where.not(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-M drop column process

---

Changelog: deprecated

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #268154 (closed)