Fix dismissal_reason not being updated in AutoDismissService

What does this MR do and why?

Fix dismissal_reason not being updated in AutoDismissService.

Vulnerabilities::Reads::UpsertService does not perform updates when FF turn_off_vulnerability_read_create_db_trigger_function is disabled: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/vulnerabilities/reads/upsert_service.rb#L35. This is causing an issue where dismissal_reason is not being updated.

The tests assume the FF is enabled by default, so they didn't uncover it.

The fix follows the same pattern as in BulkDismissService.

References

How to set up and validate locally

1. Enable the feature flag auto_dismiss_vulnerability_policies
2. Create a project
3. Create an auto-dismiss policy:

   vulnerability_management_policy:
   - name: Auto-dismiss acceptable secrets
     description: Auto-dismiss secrets
     enabled: true
     actions:
     - type: auto_dismiss
       dismissal_reason: not_applicable
     rules:
     - type: detected
       criteria:
       - type: file_path
         value: ".env"

4. Add .gitlab-ci.yml with secret detection

   include:
   - template: Jobs/Secret-Detection.latest.gitlab-ci.yml    

5. Add two secrets in the main branch:
   1. .env:

      AWS_TOKEN=AKIAZYONPI3G4JNCCWGQ

6. Wait until the pipeline on main finishes
7. Use GraphQL explorer to query the dismissalReason flag for the vulnerabilities and verify that dismissalReason is NOT_APPLICABLE:

   {
     project(
       fullPath: "<project-path>"
     ) {
       vulnerabilities {
         nodes {
           title
           dismissalReason
           policyAutoDismissed
         }
       }
     }
   }

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Related to #581377