Add the ignore_column on Corpus project_id

What does this MR do and why?

1. The column project_id of Corpus table is ignored.
2. Set it to remove after next milestone
3. The Corpus table is behind the feature flag and empty so removing the column should not cause downtime.

How to set up and validate locally

1. Pull the branch and run rails db:migrate.
2. You can try creating a corpus using AppSec::Fuzzing::Coverage::Corpus.create(package_id: any_package_id)

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 #343971