Add migration to backfill solution to vulnerabilities from findings

What does this MR do and why?

We are moving the solution field from vulnerability_occurrences to vulnerabilities since a solution will be the same on all branches. This MR adds a background migration to backfill the field using the vulnerability_occurrences table.

This MR is step 3 of the process:

1. Add the solution column to vulnerabilities: !205203 (merged)
2. Set the solution when new vulnerabilities are created: !210401 (merged)
3. Backfill solution from vulnerability_occurrences records: This MR 👈
4. Ignore column on Vulnerability::Finding model
5. Drop column from vulnerability_occurrences table
6. Remove ignore_column statement

References

Relates to: #556208

SQL

https://console.postgres.ai/gitlab/gitlab-production-sec/sessions/45029/commands/138204

UPDATE
  vulnerabilities
SET
  solution = vulnerability_occurrences.solution
FROM
  vulnerability_occurrences
WHERE
  vulnerability_occurrences.vulnerability_id IN (SELECT "vulnerabilities"."id" FROM "vulnerabilities" WHERE "vulnerabilities"."id" BETWEEN 1000000 AND 1001000 AND "vulnerabilities"."id" >= 1001000 AND "vulnerabilities"."id" < 1001100) AND
  vulnerability_occurrences.vulnerability_id = vulnerabilities.id AND
  vulnerabilities.solution IS NULL AND
  vulnerability_occurrences.solution IS NOT NULL

Screenshots or screen recordings

Before	After

How to set up and validate locally

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