Resolve "Fix state update in FindOrCreateFromSecurityFindingService when turn_off_vulnerability_read_create_db_trigger_function disabled"

What does this MR do and why?

This code change introduces a feature flag to control how vulnerability dismissal reasons are stored in the database.

When the feature flag is enabled, the system uses a new service to handle dismissal reasons. When disabled, it falls back to the old method of directly updating the vulnerability record.

This fixes the issue when FF is disabled, and the state is updated, the new state is set in the record because the UpsertService returns without running if FF is disabled

References

How to set up and validate locally

MR acceptance checklist

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

Related to #555983