Investigate removal of requires_new in Vulnerabilities::CreateService

Why are we doing this work

We need to investigate removal of requires_new in Vulnerabilities::CreateService. This can cause subtransactions as described in https://gitlab.com/gitlab-com/runbooks/-/blob/master/docs/patroni/postgresql-subtransactions.md. This has come to light especially with the inclusion of this code for Vulnerabilities::FindOrCreateFromSecurityFindingService.

Relevant links

Issue created due to subtransaction

Implementation plan

1. Remove requires_new: true from transaction start in Vulnerabilities::CreateService
2. Ensure no regressions
3. Reschedule MigrateVulnerabilitiesFeedbackToVulnerabilitiesStateTransition