Finalize finding_id backfilling and make `finding_id` NOT NULL

What does this MR do and why?

This MR finalizes two batched background migrations and makes vulnerabilities.finding_id column NOT NULL.

How to set up and validate locally

This should generally work. The only exception I found is that if your _ci databases are out of sync with the main ones. To fix this you should:

1. DROP DATABASE gitlabhq_development_ci
2. DROP DATABASE gitlabhq_test_ci
3. bundle exec dev:copy_db:ci

Afterwards the migration should pass.

Related to #437129 (closed)