Remove always-null fields from Vulnerabilities

Why are we doing this work

As part of a spike issue, it surfaced that certain fields in Vulnerability are always null.

We'd like to remove them to simplify the DB and rails model.

Relevant links

Fields in Vulnerability that are always NULL:

• milestone_id - https://explain.depesz.com/s/59td (ignore MR, drop MR)
• epic_id - https://explain.depesz.com/s/e2FG (ignore MR, drop MR)
• updated_by_id - https://explain.depesz.com/s/hYHj (ignore MR, drop MR)
• last_edited_by_id - https://explain.depesz.com/s/hYHj (ignore MR, drop MR)
• start_date - https://explain.depesz.com/s/hYHj (ignore MR, drop MR)
• due_date - https://explain.depesz.com/s/hYHj (ignore MR, drop MR)
• last_edited_at - https://explain.depesz.com/s/hYHj (ignore MR, drop MR)
• start_date_sourcing_milestone_id - https://explain.depesz.com/s/gbWY (ignore MR, drop MR)
• due_date_sourcing_milestone_id - https://explain.depesz.com/s/gbWY (ignore MR, drop MR)

Implementation plan

• Prepare MRs for the 3-milestone process