fix: Use gitlab_sec for vuln_remediation bulk insert

What does this MR do and why?

This was missed when migrating vulnerability_remediations to gitlab_sec but the connection proxy should ensure we are fetching from the right DB.

Relates to #481330

Looks like it's the only one thankfully 🎉 :

❯ rg 'class .+< ApplicationRecord' ee/app/services/security/**/*.rb
ee/app/services/security/ingestion/tasks/ingest_remediations.rb
21:        class RemediationBulkInsertProxy < ApplicationRecord

❯ 

MR acceptance checklist

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