Use database for dependency export

What does this MR do and why?

The current dependency export reads the dependency_files property from gl-dependency-scanning-report.json. This property is removed from newer versions of the security report schema. So, analyzers will no longer be populating it soon. To ensure the dependency list export continues working, we need to read this data from the database instead.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the feature flag: echo 'Feature.enable(:use_database_for_dependency_export)' | bundle exec rails c
2. Find or create a project which has dependencies
3. Go to the dependency list for the project
4. Click the export button
5. Verify that export contains expected content